Sayyiduna Abdullah bin Amr (may Allah be pleased with him) states: On the day of Arafah, the supplication the Prophet (peace and blessings be upon him) recited most often was: "There is no god but Allah Almighty alone; He has no partner. To Him alone belongs the dominion, to Him alone belongs all praise, and He has power over all things."
Ahmad recorded it, and its narrators have been declared trustworthy (muwaththaqun).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5550