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.