Yahya related to me from Malik from Zihad ibn Abi Ziyad the mawlaof Abdullah ibn Ayyash ibn Abi Rabia from Talha ibn Ubaydullah ibnKariyz that the Messenger of Allah, may Allah bless him and grant himpeace, 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 'Thereis no god but Allah, alone, without partner.'"
USC-MSA web (English) reference: Book 20, Hadith 255
Arabic reference: Book 20, Hadith 951