Reference : Hisn al-Muslim 237

خير الدعاء دعاء يوم عرفة ، وخيرُ ما قلت أنا والنبيُّون من قبلي :
لا إله إلا الله وحدهُ
لا شريك لهُ ،
لهُ الملكُ ولهُ الحمدُ
وهو على كل شيء قدير.
The Prophet (ﷺ) said: The best invocation is that of the Day of Arafat, and the best that anyone can say is what I and the Prophets before me have said:

Lā 'ilāha 'illallāhu 
waḥdahu lā sharīka lahu, 
lahul-mulku wa lahul-ḥamdu 
wa huwa `alā kulli shay'in qadīr.

None has the right to be worshipped but Allah 
Alone, Who has no partner. 
His is the dominion and His is the praise,
and He is Able to do all things.

Reference:
At-Tirmidhi. Al-Albani graded it good in Sahih At-Tirmidhi 3/184, and also 
Silsilatul-'Ahadith As-Sahihah 4/6.

Chapter: The Day of 'Arafah