The prophet peace be upon him said:

«خير الدعاء دعاء يوم عرفة، وخير ما قلت أنا والنبيون من قبلي لا إله إلا الله وحده لا شريك له، له الملك وله الحمد وهو على كل شيء قدير»

«The best supplication is on the Day of Arafat and the best supplication said by me and the prophets before me: There is no God but Allah alone, He has no partner. To Allah is the dominion, and to Him is the praise and He is Capable of all things»