The Prophet 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 is, 'There is no god but Allah alone, with no partner. To Him belongs the dominion and to Him belongs all praise, and He has power over all things.'"
حدثنا أبو عمرو مسلم بن عمرو الحذاء المديني قال حدثني عبد الله بن نافع عن حماد بن أبي حميد
أن النبي ﷺ قال 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
[Its chain of transmission is daif]. Ahmad reported it in 2/210, hadith 6961, from the hadith of Ibn Abu Humayd, through him. Hammad bin Abu Humayd is daif, as noted previously at 3561. The hadith has daif corroborating narrations in Malik (Muwatta 1/215, hadith 501, and 1/22, 423, hadith 974) and elsewhere.