Maliks Muwatta Book 015, Hadith Number 032.

Section : Dua (Supplication).

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.)"