user6925: For what reason did Buddha rejected Upanishad and Vedas?

user8157: Reference to the Upanishads does not exist in the Pali suttas. Only reference to the Four Vedas is found in the Pali suttas. The Buddha was not aware of the Upanishads and had no interest in the superstitious ideas found in the Four Vedas. 
The Buddha found the complete cessation of suffering, which is not found in the Vedas. The complete cessation of suffering occurs with the utter destruction of "self-view", which includes the destruction of ideas about "Brahma" and "Atman". 

user471: A few Upanishads are pre-Buddhist but the majority came later. Most of the Vedas were however pre-Buddhist.
The Buddha definitely rejected the idea of the Atman (an indivisible indestructible eternal self at the core of beings) and he also rejected the concept of eternalism. Atman and eternalism are core doctrines of the Upanishads.
Not just this, there are rituals that are either contained in or derived from the Vedas like the following (from this site):

In its simplest form, which is still practiced by many Brahmanas and
  members of the higher castes, it is practiced just before the dawn by
  standing in a water body such as a river or a water tank and offering
  water (tarpana) to the Sun as it rises in the east with a prayer and a
  salutation (vandanam), which makes the sacrifice both an external and
  an internal cleansing worship.

In the Sigalovada Sutta, the Buddha rejected the ritualism behind the Vedic bath prayer ceremony (as above) practised by a young householder by the name of Sigala, and put a new spin on it, according to the Buddha Dhamma.
Also, from the Canki Sutta, the Buddha hints that even the learned brahmans themselves are unsure whether the Vedas are true, by their own empirical validation:

"And among the brahman seers of the past, the creators of the hymns,
  the composers of the hymns — those ancient hymns, sung, repeated, &
  collected, which brahmans at present still sing, still chant,
  repeating what was said, repeating what was spoken — i.e., Atthaka,
  Vamaka, Vamadeva, Vessamitta, Yamataggi, Angirasa, Bharadvaja,
  Vasettha, Kassapa & Bhagu: was there even one of these who said, 'This
  we know; this we see; only this is true; anything else is worthless?'"
"No, Master Gotama."
"So then, Bharadvaja, it seems that there isn't among the brahmans
  even one brahman who says, 'This I know; this I see; only this is
  true; anything else is worthless.' And there hasn't been among the
  brahmans even one teacher or teacher's teacher back through seven
  generations who said, 'This I know; this I see; only this is true;
  anything else is worthless.' And there hasn't been among the brahman
  seers of the past, the creators of the hymns, the composers of the
  hymns... even one who said, 'This we know; this we see; only this is
  true; anything else is worthless.' Suppose there were a row of blind
  men, each holding on to the one in front of him: the first one doesn't
  see, the middle one doesn't see, the last one doesn't see. In the same
  way, the statement of the brahmans turns out to be a row of blind men,
  as it were: the first one doesn't see, the middle one doesn't see, the
  last one doesn't see. So what do you think, Bharadvaja: this being the
  case, doesn't the conviction of the brahmans turn out to be
  groundless?"

In the Sangarava Sutta, the Buddha rejected purification rites using water.
In the Sundarika Sutta, the Buddha rejected caste by birth.
In the Paccha-bhumika Sutta, the Buddha rejected the use of prayers to help the deceased go to heavan.
In the Samannaphala Sutta, the Buddha forbade his monks from doing fire oblations or fire sacrifices, practising astrology, reading omens, calculating auspicious dates for marriages, consecrating sites for construction and worshipping the Sun. Some of these practices are religious, while others are cultural.

user10100: Because nibbāna of Upanishad and Vedas is sassata-diṭṭhi (eternalism) in brahmmajālasutta. But nibbāna of buddhism is enlightened by ariya at the finish line (enlightenment) of the middle way, that is in the midst of whole extreme teaching around the universe, such as eternalism,  annihilationism, egoism, altruism,  theistic determinism, determinism that only past-life's karmma is cause of present life (i can't find the correct word of this view), accidentalism, nihilism, Marxism, democrazy, communism, science, capitalism, etc.
What is the middle way?
The middle way is ariya-magga (The Noble Eightfold Path), the best balanced teaching which must leads to the complete and perfect whole suffering cessation (anupādisesa-nibbānadhātu). The middle way must have these whole properties as their results, not just some properties:

Dhammacakkappavattanasutta:
By avoiding these two extremes, O bhikkhus, the Tathāgata has gained
the knowledge of the middle path which leads to insight, which leads
to wisdom, which conduces to calm, to knowledge, to the Sambodhi, to
Nirvāna.
....
‘This, O bhikkhus, is the Noble Truth of the Cessation of Suffering: (it ceases with) the complete cessation of this thirst -- a
cessation which consists in the absence of every passion -- with the
abandoning of this thirst, with the doing-away with it, with the
deliverance from it, with the destruction of desire.
...
‘And this knowledge and insight arose in my mind: "The emancipation of my mind cannot be lost; this is my last birth; hence I shall not be born again!"’
ādittasutta (ādittapariyāyasutta):
“Monks, the learned, noble disciple seeing it thus turns from the eye and forms, eye-consciousness, eye-contact and whatever feelings,
pleasant unpleasant or neither unpleasant nor pleasant born of
eye-contact, he turns from that too  re  He turns from the mind,
deas, mind-consciousness, mind-contact and whatever feelings, pleasant
unpleasant or neither unpleasant nor pleasant born of mind-contact, he
turns from that too. Turning loses interest. Losing interest is
released. Released knowledge arises to him, I am released, birth is
destroyed, the holy life is lived to the end, duties are done, I have
nothing more to wish.”
The Blessed One said thus and those monks delighted in the words of the Blessed One.
When this exposition was done about a thousand monks released their minds from desires without anything remaining.
mahāsatipaṭṭhānasutta:
"These are the single gold paths to the purification of beings, to the overcoming of sorrow & lamentation, to the disappearance of pain & distress, to the attainment of the right method, & to the realization of Unbinding — Summary are the foundations of mindfulness.

Tipitaka studing that heritage from Arahanta, tipitaka reciting, and tipitaka memorizing his tipitaka knowledge, these 3 things make the buddhist practitioner understands tipitaka.

6925: <p>For what reason did Buddha rejected Upanishad and Vedas?</p>
 8157: <p>Reference to the Upanishads does not exist in the Pali suttas. Only reference to the Four Vedas is found in the Pali suttas. The Buddha was not aware of the Upanishads and had no interest in the superstitious ideas found in the Four Vedas. </p>

<p>The Buddha found the complete cessation of suffering, which is not found in the Vedas. The complete cessation of suffering occurs with the utter destruction of "self-view", which includes the destruction of ideas about "Brahma" and "Atman". </p>
 471: <p>A few Upanishads are pre-Buddhist but the majority came later. Most of the Vedas were however pre-Buddhist.</p>

<p>The Buddha definitely rejected the idea of the Atman (an indivisible indestructible eternal self at the core of beings) and he also rejected the concept of eternalism. Atman and eternalism are core doctrines of the Upanishads.</p>

<p>Not just this, there are rituals that are either contained in or derived from the Vedas like the following (from <a href="http://www.hinduwebsite.com/rites.asp" rel="nofollow noreferrer">this site</a>):</p>

<blockquote>
  <p>In its simplest form, which is still practiced by many Brahmanas and
  members of the higher castes, it is practiced just before the dawn by
  standing in a water body such as a river or a water tank and offering
  water (tarpana) to the Sun as it rises in the east with a prayer and a
  salutation (vandanam), which makes the sacrifice both an external and
  an internal cleansing worship.</p>
</blockquote>

<p>In the <a href="https://www.accesstoinsight.org/tipitaka/dn/dn.31.0.ksw0.html" rel="nofollow noreferrer">Sigalovada Sutta</a>, the Buddha rejected the ritualism behind the Vedic bath prayer ceremony (as above) practised by a young householder by the name of Sigala, and put a new spin on it, according to the Buddha Dhamma.</p>

<p>Also, from the <a href="https://www.accesstoinsight.org/tipitaka/mn/mn.095x.than.html" rel="nofollow noreferrer">Canki Sutta</a>, the Buddha hints that even the learned brahmans themselves are unsure whether the Vedas are true, by their own empirical validation:</p>

<blockquote>
  <p>"And among the brahman seers of the past, the creators of the hymns,
  the composers of the hymns — those ancient hymns, sung, repeated, &amp;
  collected, which brahmans at present still sing, still chant,
  repeating what was said, repeating what was spoken — i.e., Atthaka,
  Vamaka, Vamadeva, Vessamitta, Yamataggi, Angirasa, Bharadvaja,
  Vasettha, Kassapa &amp; Bhagu: was there even one of these who said, 'This
  we know; this we see; only this is true; anything else is worthless?'"</p>
  
  <p>"No, Master Gotama."</p>
  
  <p>"So then, Bharadvaja, it seems that there isn't among the brahmans
  even one brahman who says, 'This I know; this I see; only this is
  true; anything else is worthless.' And there hasn't been among the
  brahmans even one teacher or teacher's teacher back through seven
  generations who said, 'This I know; this I see; only this is true;
  anything else is worthless.' And there hasn't been among the brahman
  seers of the past, the creators of the hymns, the composers of the
  hymns... even one who said, 'This we know; this we see; only this is
  true; anything else is worthless.' Suppose there were a row of blind
  men, each holding on to the one in front of him: the first one doesn't
  see, the middle one doesn't see, the last one doesn't see. <strong>In the same
  way, the statement of the brahmans turns out to be a row of blind men,
  as it were: the first one doesn't see, the middle one doesn't see, the
  last one doesn't see. So what do you think, Bharadvaja: this being the
  case, doesn't the conviction of the brahmans turn out to be
  groundless?"</strong></p>
</blockquote>

<p>In the <a href="https://suttacentral.net/sn7.21/en/sujato" rel="nofollow noreferrer">Sangarava Sutta</a>, the Buddha rejected purification rites using water.</p>

<p>In the <a href="https://suttacentral.net/sn7.9/en/sujato" rel="nofollow noreferrer">Sundarika Sutta</a>, the Buddha rejected caste by birth.</p>

<p>In the <a href="https://www.accesstoinsight.org/tipitaka/sn/sn42/sn42.006.than.html" rel="nofollow noreferrer">Paccha-bhumika Sutta</a>, the Buddha rejected the use of prayers to help the deceased go to heavan.</p>

<p>In the <a href="https://www.accesstoinsight.org/tipitaka/dn/dn.02.0.than.html#livelihood" rel="nofollow noreferrer">Samannaphala Sutta</a>, the Buddha forbade his monks from doing fire oblations or fire sacrifices, practising astrology, reading omens, calculating auspicious dates for marriages, consecrating sites for construction and worshipping the Sun. Some of these practices are religious, while others are cultural.</p>
 10100: <p>Because nibbāna of Upanishad and Vedas is sassata-diṭṭhi (eternalism) in <a href="http://84000.org/tipitaka/read/v.php?B=09&amp;A=1&amp;Z=1071&amp;eng=metta_e" rel="nofollow noreferrer">brahmmajālasutta</a>. But nibbāna of buddhism is enlightened by ariya at <strong>the finish line (enlightenment) of the middle way</strong>, that is <strong>in the midst of whole extreme teaching around the universe</strong>, such as eternalism,  annihilationism, egoism, altruism,  theistic determinism, determinism that only past-life's karmma is cause of present life (i can't find the correct word of this view), accidentalism, nihilism, Marxism, democrazy, communism, science, capitalism, etc.</p>
<h2>What is the middle way?</h2>
<p>The middle way is <a href="http://www.84000.org/tipitaka/pitaka_item/v.php?B=10&amp;A=6257&amp;Z=6764&amp;eng=metta_e2" rel="nofollow noreferrer">ariya-magga</a> (The Noble Eightfold Path), <strong>the best balanced teaching which must leads to the complete and perfect whole suffering cessation (anupādisesa-nibbānadhātu)</strong>. The middle way must have these whole properties as their results, not just some properties:</p>
<blockquote>
<p><a href="http://www.84000.org/tipitaka/read/v.php?B=04&amp;A=355&amp;Z=445&amp;eng=metta_e" rel="nofollow noreferrer">Dhammacakkappavattanasutta</a>:</p>
<p>By avoiding these two extremes, O bhikkhus, the Tathāgata has gained
the knowledge of <strong>the middle path which leads to insight, which leads
to wisdom, which conduces to calm, to knowledge, to the Sambodhi, to
Nirvāna.</strong></p>
<p>....</p>
<p>‘This, O bhikkhus, is the Noble Truth of the Cessation of Suffering: (it ceases with) the complete cessation of this thirst -- a
cessation which consists in the absence of every passion -- with the
abandoning of this thirst, with the doing-away with it, with the
deliverance from it, with the destruction of desire.</p>
<p>...</p>
<p>‘And this knowledge and insight arose in my mind: &quot;The emancipation of my mind cannot be lost; this is my last birth; hence I shall not be born again!&quot;’</p>
<p><a href="http://www.84000.org/tipitaka/read/v.php?B=04&amp;A=355&amp;Z=445&amp;eng=metta_e" rel="nofollow noreferrer">ādittasutta</a> (ādittapariyāyasutta):</p>
<p>“Monks, the learned, noble disciple seeing it thus turns from the eye and forms, eye-consciousness, eye-contact and whatever feelings,
pleasant unpleasant or neither unpleasant nor pleasant born of
eye-contact, he turns from that too  re  He turns from the mind,
deas, mind-consciousness, mind-contact and whatever feelings, pleasant
unpleasant or neither unpleasant nor pleasant born of mind-contact, he
turns from that too. Turning loses interest. Losing interest is
released. Released knowledge arises to him, I am released, birth is
destroyed, the holy life is lived to the end, duties are done, I have
nothing more to wish.”</p>
<p>The Blessed One said thus and those monks delighted in the words of the Blessed One.</p>
<p>When this exposition was done about a thousand monks released their minds from desires without anything remaining.</p>
<p><a href="http://www.84000.org/tipitaka/pitaka_item/v.php?B=10&amp;A=6257&amp;Z=6764&amp;eng=metta_e2" rel="nofollow noreferrer">mahāsatipaṭṭhānasutta</a>:</p>
<p>&quot;These are the single gold paths to the purification of beings, to the overcoming of sorrow &amp; lamentation, to the disappearance of pain &amp; distress, to the attainment of the right method, &amp; to the realization of Unbinding — Summary are the foundations of mindfulness.</p>
</blockquote>
<p>Tipitaka studing that heritage from Arahanta, tipitaka reciting, and tipitaka memorizing his tipitaka knowledge, these 3 things make the buddhist practitioner understands tipitaka.</p>
