Allah's Messenger ﷺ said: The best supplication is the supplication of the Day of Arafah, and the best of what I and the prophets before me have said (on that day) is:

لَآ اِلٰهَ اِلَّا اللّٰهُ وَحْدَهٗ لَا شَرِيْكَ لَهٗ ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلٰى كُلِّ شَيْءٍ قَدِيْرٌ

There is no deity except Allah; He is alone and has no partner. His is the dominion and His is all praise, and He has full power over all things.

Hasan li-ghairihi, Sunan at-Tirmidhi, no. (3585), from the hadith of Abdullah bin Amr, Fadl Ashr Dhi al-Hijjah of at-Tabarani, no. (51), from the hadith of Ali, the wording is theirs, Muwatta' Malik, ed. Abd al-Baqi (1/214), from the hadith of Talhah as mursal, and al-Albani graded it Hasan in “as-Sahihah” no. (1503).