Dua #1102•At-Tirmidhi. Al-Albani graded it good in Sahih At-Tirmidhi 3/184, and also Silsilatul-'Ahadith As-Sahihah 4/6.

The Day of Arafah

Hisnul Muslim
#1102

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.

Transliteration

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

Translation

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.