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.)"
Muwatta Malik Book 15, Hadith 34