Muwatta Malik, Book 20, Hadith 951 (In-Book) ⓘ

Yahya related to me from Malik from Zihad ibn Abi Ziyad the mawla of Abdullah ibn Ayyash ibn Abi Rabia from Talha ibn Ubaydullah ibn Kariyz that the Messenger of Allah, may Allah bless him and grant him peace, said, "The most excellent dua is the dua on the Day of Arafa, and the best of what I and the prophets before me have said, is 'There is no god but Allah, alone, without partner.'"

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زِيَادِ بْنِ أَبِي زِيَادٍ، مَوْلَى عَبْدِ اللَّهِ بْنِ عَيَّاشِ بْنِ أَبِي رَبِيعَةَ عَنْ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ بْنِ كَرِيزٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ أَفْضَلُ الدُّعَاءِ دُعَاءُ يَوْمِ عَرَفَةَ وَأَفْضَلُ مَا قُلْتُ أَنَا وَالنَّبِيُّونَ مِنْ قَبْلِي لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ ‏" ‏ ‏.‏