user14387: What is the (commentary) source for Buddha's five-part daily schedule, as mentioned at The Buddha's Daily Routine (Buddhanet) and A Day in the Life of the Buddha (YouTube video, Bhikkhu Bodhi).

user10100: It's from Tipitaka like ruben2020 answered.
The commentary teachers are the Tipitaka Memorizer, so they concluded the Buddha's normal daily schedule like below...
sīlakkhandhavagga,aṭṭhakathā 45 paṇṇe

sātthakaṃyeva pana bhagavato kiccaṃ hoti. taṃ pañcavidhaṃ –
purebhattakiccaṃ, pacchābhattakiccaṃ, purimayāmakiccaṃ,
majjhimayāmakiccaṃ, pacchimayāmakiccanti.
tatridaṃ purebhattakiccaṃ –
bhagavā hi pātova uṭṭhāya upaṭṭhākānuggahatthaṃ sarīraphāsukatthañca
mukhadhovanādisarīraparikammaṃ katvā yāva bhikkhācāravelā tāva
vivittāsane vītināmetvā, bhikkhācāravelāyaṃ nivāsetvā kāyabandhanaṃ
bandhitvā cīvaraṃ pārupitvā pattamādāya kadāci ekako, kadāci
bhikkhusaṅghaparivuto, gāmaṃ vā nigamaṃ vā piṇḍāya pavisati; kadāci
pakatiyā, kadāci anekehi pāṭihāriyehi vattamānehi. seyyathidaṃ,
piṇḍāya pavisato lokanāthassa purato purato gantvā mudugatavātā
pathaviṃ sodhenti, valāhakā udakaphusitāni muñcantā magge reṇuṃ
vūpasametvā upari vitānaṃ hutvā tiṭṭhanti, apare vātā pupphāni
upasaṃharitvā magge okiranti, unnatā bhūmippadesā onamanti, onatā
unnamanti, pādanikkhepasamaye samāva bhūmi hoti, sukhasamphassāni
padumapupphāni vā pāde sampaṭicchanti. indakhīlassa anto ṭhapitamatte
dakkhiṇapāde sarīrato chabbaṇṇarasmiyo nikkhamitvā
suvaṇṇarasapiñjarāni viya citrapaṭaparikkhittāni viya ca
pāsādakūṭāgārādīni alaṅkarontiyo ito cito ca dhāvanti,
hatthiassavihaṅgādayo sakasakaṭṭhānesu ṭhitāyeva madhurenākārena
saddaṃ karonti, tathā bherivīṇādīni tūriyāni manussānañca kāyūpagāni
ābharaṇāni. tena saññāṇena manussā jānanti – ‘‘ajja bhagavā idha
piṇḍāya paviṭṭho’’ti. te sunivatthā supārutā gandhapupphādīni ādāya
gharā nikkhamitvā antaravīthiṃ paṭipajjitvā bhagavantaṃ
gandhapupphādīhi sakkaccaṃ pūjetvā vanditvā – ‘‘amhākaṃ, bhante, dasa
bhikkhū, amhākaṃ vīsati, paññāsaṃ…pe.… sataṃ dethā’’ti yācitvā
bhagavatopi pattaṃ gahetvā āsanaṃ paññapetvā sakkaccaṃ piṇḍapātena
paṭimānenti. bhagavā katabhattakicco tesaṃ sattānaṃ cittasantānāni
oloketvā tathā dhammaṃ deseti, yathā keci saraṇagamanesu
patiṭṭhahanti, keci pañcasu sīlesu, keci
sotāpattisakadāgāmianāgāmiphalānaṃ aññatarasmiṃ; keci pabbajitvā
aggaphale arahatteti. evaṃ mahājanaṃ anuggahetvā uṭṭhāyāsanā vihāraṃ
gacchati.
tattha gantvā maṇḍalamāḷe paññattavarabuddhāsane nisīdati, bhikkhūnaṃ
bhattakiccapariyosānaṃ āgamayamāno. tato bhikkhūnaṃ
bhattakiccapariyosāne upaṭṭhāko bhagavato nivedeti. atha bhagavā
gandhakuṭiṃ pavisati. idaṃ tāva purebhattakiccaṃ.
atha bhagavā evaṃ katapurebhattakicco gandhakuṭiyā upaṭṭhāne nisīditvā
pāde pakkhāletvā pādapīṭhe ṭhatvā bhikkhusaṅghaṃ ovadati –
‘‘bhikkhave, appamādena sampādetha, dullabho buddhuppādo lokasmiṃ,
dullabho manussattapaṭilābho, dullabhā sampatti, dullabhā pabbajjā,
dullabhaṃ saddhammassavana’’nti. tattha keci bhagavantaṃ kammaṭṭhānaṃ
pucchanti. bhagavāpi tesaṃ cariyānurūpaṃ kammaṭṭhānaṃ deti. tato
sabbepi bhagavantaṃ vanditvā attano attano rattiṭṭhānadivāṭṭhānāni
gacchanti. keci araññaṃ, keci rukkhamūlaṃ, keci pabbatādīnaṃ
aññataraṃ, keci cātumahārājikabhavanaṃ…pe.… keci vasavattibhavananti.
tato bhagavā gandhakuṭiṃ pavisitvā sace ākaṅkhati, dakkhiṇena passena
sato sampajāno muhuttaṃ sīhaseyyaṃ kappeti. atha samassāsitakāyo
vuṭṭhahitvā dutiyabhāge lokaṃ voloketi. tatiyabhāge yaṃ gāmaṃ vā
nigamaṃ vā upanissāya viharati tattha mahājano purebhattaṃ dānaṃ datvā
pacchābhattaṃ sunivattho supāruto gandhapupphādīni ādāya vihāre
sannipatati. tato bhagavā sampattaparisāya anurūpena pāṭihāriyena
gantvā dhammasabhāyaṃ paññattavarabuddhāsane nisajja dhammaṃ deseti
kālayuttaṃ samayayuttaṃ, atha kālaṃ viditvā parisaṃ uyyojeti, manussā
bhagavantaṃ vanditvā pakkamanti. idaṃ pacchābhattakiccaṃ.
so evaṃ niṭṭhitapacchābhattakicco sace gattāni osiñcitukāmo hoti,
buddhāsanā vuṭṭhāya nhānakoṭṭhakaṃ pavisitvā upaṭṭhākena
paṭiyāditaudakena gattāni utuṃ gaṇhāpeti. upaṭṭhākopi buddhāsanaṃ
ānetvā gandhakuṭipariveṇe paññapeti. bhagavā surattadupaṭṭaṃ nivāsetvā
kāyabandhanaṃ bandhitvā uttarāsaṅgaṃ ekaṃsaṃ karitvā tattha gantvā
nisīdati ekakova muhuttaṃ paṭisallīno, atha bhikkhū tato tato āgamma
bhagavato upaṭṭhānaṃ āgacchanti.
tattha ekacce pañhaṃ pucchanti, ekacce kammaṭṭhānaṃ, ekacce
dhammassavanaṃ yācanti. bhagavā tesaṃ adhippāyaṃ sampādento
purimayāmaṃ vītināmeti. idaṃ purimayāmakiccaṃ.
purimayāmakiccapariyosāne pana bhikkhūsu bhagavantaṃ vanditvā
pakkantesu sakaladasasahassilokadhātudevatāyo okāsaṃ labhamānā
bhagavantaṃ upasaṅkamitvā pañhaṃ pucchanti, yathābhisaṅkhataṃ antamaso
caturakkharampi. bhagavā tāsaṃ devatānaṃ pañhaṃ vissajjento
majjhimayāmaṃ vītināmeti. idaṃ majjhimayāmakiccaṃ.
pacchimayāmaṃ pana tayo koṭṭhāse katvā purebhattato paṭṭhāya nisajjāya
pīḷitassa sarīrassa kilāsubhāvamocanatthaṃ ekaṃ koṭṭhāsaṃ caṅkamena
vītināmeti. dutiyakoṭṭhāse gandhakuṭiṃ pavisitvā dakkhiṇena passena
sato sampajāno sīhaseyyaṃ kappeti. tatiyakoṭṭhāse paccuṭṭhāya
nisīditvā purimabuddhānaṃ santike dānasīlādivasena
katādhikārapuggaladassanatthaṃ buddhacakkhunā lokaṃ voloketi. idaṃ
pacchimayāmakiccaṃ.


user471: We can find mentions of morning, midday or noon and afternoon in AN 3.19.

In the same way, a mendicant who has three factors is unable to
acquire more skillful qualities or to increase the skillful qualities
they’ve already acquired. What three? It’s when a mendicant doesn’t
carefully apply themselves to a meditation subject as a foundation of
immersion in the morning, at midday, and in the afternoon.
AN 3.19

For the three watches of the night, we can find it in MN 53. The parts in square brackets are comments from the translator.

"And how is the disciple of the noble ones devoted to wakefulness?
There is the case where a disciple of the noble ones, sitting & pacing
back & forth, cleanses his mind of any qualities that would hold the
mind in check. During the first watch of the night [dusk to 10
p.m.], sitting & pacing back & forth, he cleanses his mind of any
qualities that would hold the mind in check. During the second watch
of the night [10 p.m. to 2 a.m.], reclining on his right side, he
takes up the lion's posture, one foot placed on top of the other,
mindful, alert, with his mind set on getting up [either as soon as he
awakens or at a particular time]. During the last watch of the
night [2 a.m. to dawn], sitting & pacing back & forth, he cleanses
his mind of any qualities that would hold the mind in check. This is
how the monk is devoted to wakefulness.
MN 53


14387: <p>What is the (commentary) source for Buddha's five-part daily schedule, as mentioned at <a href="http://www.buddhanet.net/e-learning/buddhism/lifebuddha/26lbud.htm" rel="nofollow noreferrer">The Buddha's Daily Routine</a> (Buddhanet) and <a href="https://www.youtube.com/watch?v=7J17nzlmPnE" rel="nofollow noreferrer">A Day in the Life of the Buddha</a> (YouTube video, Bhikkhu Bodhi).</p>
 10100: <p>It's from Tipitaka like ruben2020 answered.</p>
<p>The commentary teachers are the Tipitaka Memorizer, so they concluded the <a href="https://5000y.men/?rm.r.67.45.0.4." rel="nofollow noreferrer">Buddha's normal daily schedule</a> like below...</p>
<p>sīlakkhandhavagga,aṭṭhakathā 45 paṇṇe</p>
<blockquote>
<p>sātthakaṃyeva pana bhagavato kiccaṃ hoti. taṃ pañcavidhaṃ –
purebhattakiccaṃ, pacchābhattakiccaṃ, purimayāmakiccaṃ,
majjhimayāmakiccaṃ, pacchimayāmakiccanti.</p>
<p>tatridaṃ purebhattakiccaṃ –</p>
<p>bhagavā hi pātova uṭṭhāya upaṭṭhākānuggahatthaṃ sarīraphāsukatthañca
mukhadhovanādisarīraparikammaṃ katvā yāva bhikkhācāravelā tāva
vivittāsane vītināmetvā, bhikkhācāravelāyaṃ nivāsetvā kāyabandhanaṃ
bandhitvā cīvaraṃ pārupitvā pattamādāya kadāci ekako, kadāci
bhikkhusaṅghaparivuto, gāmaṃ vā nigamaṃ vā piṇḍāya pavisati; kadāci
pakatiyā, kadāci anekehi pāṭihāriyehi vattamānehi. seyyathidaṃ,
piṇḍāya pavisato lokanāthassa purato purato gantvā mudugatavātā
pathaviṃ sodhenti, valāhakā udakaphusitāni muñcantā magge reṇuṃ
vūpasametvā upari vitānaṃ hutvā tiṭṭhanti, apare vātā pupphāni
upasaṃharitvā magge okiranti, unnatā bhūmippadesā onamanti, onatā
unnamanti, pādanikkhepasamaye samāva bhūmi hoti, sukhasamphassāni
padumapupphāni vā pāde sampaṭicchanti. indakhīlassa anto ṭhapitamatte
dakkhiṇapāde sarīrato chabbaṇṇarasmiyo nikkhamitvā
suvaṇṇarasapiñjarāni viya citrapaṭaparikkhittāni viya ca
pāsādakūṭāgārādīni alaṅkarontiyo ito cito ca dhāvanti,
hatthiassavihaṅgādayo sakasakaṭṭhānesu ṭhitāyeva madhurenākārena
saddaṃ karonti, tathā bherivīṇādīni tūriyāni manussānañca kāyūpagāni
ābharaṇāni. tena saññāṇena manussā jānanti – ‘‘ajja bhagavā idha
piṇḍāya paviṭṭho’’ti. te sunivatthā supārutā gandhapupphādīni ādāya
gharā nikkhamitvā antaravīthiṃ paṭipajjitvā bhagavantaṃ
gandhapupphādīhi sakkaccaṃ pūjetvā vanditvā – ‘‘amhākaṃ, bhante, dasa
bhikkhū, amhākaṃ vīsati, paññāsaṃ…pe.… sataṃ dethā’’ti yācitvā
bhagavatopi pattaṃ gahetvā āsanaṃ paññapetvā sakkaccaṃ piṇḍapātena
paṭimānenti. bhagavā katabhattakicco tesaṃ sattānaṃ cittasantānāni
oloketvā tathā dhammaṃ deseti, yathā keci saraṇagamanesu
patiṭṭhahanti, keci pañcasu sīlesu, keci
sotāpattisakadāgāmianāgāmiphalānaṃ aññatarasmiṃ; keci pabbajitvā
aggaphale arahatteti. evaṃ mahājanaṃ anuggahetvā uṭṭhāyāsanā vihāraṃ
gacchati.</p>
<p>tattha gantvā maṇḍalamāḷe paññattavarabuddhāsane nisīdati, bhikkhūnaṃ
bhattakiccapariyosānaṃ āgamayamāno. tato bhikkhūnaṃ
bhattakiccapariyosāne upaṭṭhāko bhagavato nivedeti. atha bhagavā
gandhakuṭiṃ pavisati. idaṃ tāva purebhattakiccaṃ.</p>
<p>atha bhagavā evaṃ katapurebhattakicco gandhakuṭiyā upaṭṭhāne nisīditvā
pāde pakkhāletvā pādapīṭhe ṭhatvā bhikkhusaṅghaṃ ovadati –
‘‘bhikkhave, appamādena sampādetha, dullabho buddhuppādo lokasmiṃ,
dullabho manussattapaṭilābho, dullabhā sampatti, dullabhā pabbajjā,
dullabhaṃ saddhammassavana’’nti. tattha keci bhagavantaṃ kammaṭṭhānaṃ
pucchanti. bhagavāpi tesaṃ cariyānurūpaṃ kammaṭṭhānaṃ deti. tato
sabbepi bhagavantaṃ vanditvā attano attano rattiṭṭhānadivāṭṭhānāni
gacchanti. keci araññaṃ, keci rukkhamūlaṃ, keci pabbatādīnaṃ
aññataraṃ, keci cātumahārājikabhavanaṃ…pe.… keci vasavattibhavananti.
tato bhagavā gandhakuṭiṃ pavisitvā sace ākaṅkhati, dakkhiṇena passena
sato sampajāno muhuttaṃ sīhaseyyaṃ kappeti. atha samassāsitakāyo
vuṭṭhahitvā dutiyabhāge lokaṃ voloketi. tatiyabhāge yaṃ gāmaṃ vā
nigamaṃ vā upanissāya viharati tattha mahājano purebhattaṃ dānaṃ datvā
pacchābhattaṃ sunivattho supāruto gandhapupphādīni ādāya vihāre
sannipatati. tato bhagavā sampattaparisāya anurūpena pāṭihāriyena
gantvā dhammasabhāyaṃ paññattavarabuddhāsane nisajja dhammaṃ deseti
kālayuttaṃ samayayuttaṃ, atha kālaṃ viditvā parisaṃ uyyojeti, manussā
bhagavantaṃ vanditvā pakkamanti. idaṃ pacchābhattakiccaṃ.</p>
<p>so evaṃ niṭṭhitapacchābhattakicco sace gattāni osiñcitukāmo hoti,
buddhāsanā vuṭṭhāya nhānakoṭṭhakaṃ pavisitvā upaṭṭhākena
paṭiyāditaudakena gattāni utuṃ gaṇhāpeti. upaṭṭhākopi buddhāsanaṃ
ānetvā gandhakuṭipariveṇe paññapeti. bhagavā surattadupaṭṭaṃ nivāsetvā
kāyabandhanaṃ bandhitvā uttarāsaṅgaṃ ekaṃsaṃ karitvā tattha gantvā
nisīdati ekakova muhuttaṃ paṭisallīno, atha bhikkhū tato tato āgamma
bhagavato upaṭṭhānaṃ āgacchanti.</p>
<p>tattha ekacce pañhaṃ pucchanti, ekacce kammaṭṭhānaṃ, ekacce
dhammassavanaṃ yācanti. bhagavā tesaṃ adhippāyaṃ sampādento
purimayāmaṃ vītināmeti. idaṃ purimayāmakiccaṃ.</p>
<p>purimayāmakiccapariyosāne pana bhikkhūsu bhagavantaṃ vanditvā
pakkantesu sakaladasasahassilokadhātudevatāyo okāsaṃ labhamānā
bhagavantaṃ upasaṅkamitvā pañhaṃ pucchanti, yathābhisaṅkhataṃ antamaso
caturakkharampi. bhagavā tāsaṃ devatānaṃ pañhaṃ vissajjento
majjhimayāmaṃ vītināmeti. idaṃ majjhimayāmakiccaṃ.</p>
<p>pacchimayāmaṃ pana tayo koṭṭhāse katvā purebhattato paṭṭhāya nisajjāya
pīḷitassa sarīrassa kilāsubhāvamocanatthaṃ ekaṃ koṭṭhāsaṃ caṅkamena
vītināmeti. dutiyakoṭṭhāse gandhakuṭiṃ pavisitvā dakkhiṇena passena
sato sampajāno sīhaseyyaṃ kappeti. tatiyakoṭṭhāse paccuṭṭhāya
nisīditvā purimabuddhānaṃ santike dānasīlādivasena
katādhikārapuggaladassanatthaṃ buddhacakkhunā lokaṃ voloketi. idaṃ
pacchimayāmakiccaṃ.</p>
</blockquote>
 471: <p>We can find mentions of morning, midday or noon and afternoon in <a href="https://suttacentral.net/an3.19/en/sujato" rel="nofollow noreferrer">AN 3.19</a>.</p>
<blockquote>
<p>In the same way, a mendicant who has three factors is unable to
acquire more skillful qualities or to increase the skillful qualities
they’ve already acquired. What three? It’s when a mendicant doesn’t
carefully apply themselves to a meditation subject as a foundation of
immersion in the <strong>morning, at midday, and in the afternoon</strong>.<br />
<a href="https://suttacentral.net/an3.19/en/sujato" rel="nofollow noreferrer">AN 3.19</a></p>
</blockquote>
<p>For the three watches of the night, we can find it in <a href="https://www.accesstoinsight.org/tipitaka/mn/mn.053.than.html" rel="nofollow noreferrer">MN 53</a>. The parts in square brackets are comments from the translator.</p>
<blockquote>
<p>&quot;And how is the disciple of the noble ones devoted to wakefulness?
There is the case where a disciple of the noble ones, sitting &amp; pacing
back &amp; forth, cleanses his mind of any qualities that would hold the
mind in check. During the <strong>first watch of the night</strong> [dusk to 10
p.m.], sitting &amp; pacing back &amp; forth, he cleanses his mind of any
qualities that would hold the mind in check. During the <strong>second watch
of the night</strong> [10 p.m. to 2 a.m.], reclining on his right side, he
takes up the lion's posture, one foot placed on top of the other,
mindful, alert, with his mind set on getting up [either as soon as he
awakens or at a particular time]. During the <strong>last watch of the
night</strong> [2 a.m. to dawn], sitting &amp; pacing back &amp; forth, he cleanses
his mind of any qualities that would hold the mind in check. This is
how the monk is devoted to wakefulness.<br />
<a href="https://www.accesstoinsight.org/tipitaka/mn/mn.053.than.html" rel="nofollow noreferrer">MN 53</a></p>
</blockquote>
