Yahya related to me from Malik from Ziyad ibn Abi Ziyad fromTalha ibn Ubaydullah ibn Kariz that the Messenger of Allah, may Allahbless him and grant him peace, said, "The best dua is dua on the dayof Arafa, and the best thing that I or the Prophets before me havesaid is 'There is no god but Allah, alone, without any partner' (Lailaha illa'llah, wahdahu la sharika lah.)"
USC-MSA web (English) reference: Book 15, Hadith 32
Arabic reference: Book 15, Hadith 504