Muwatta Malik, Book 15, Hadith 504 (In-Book) ⓘ

Yahya related to me from Malik from Ziyad ibn Abi Ziyad from Talha ibn Ubaydullah ibn Kariz that the Messenger of Allah, may Allah bless him and grant him peace, said, "The best dua is dua on the day of Arafa, and the best thing that I or the Prophets before me have said is 'There is no god but Allah, alone, without any partner' (La ilaha illa'llah, wahdahu la sharika lah.)"

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زِيَادِ بْنِ أَبِي زِيَادٍ، 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 ‏" ‏ ‏.‏