Musannaf Ibn Abi Shayba 21:402

'Affan narrated to us, Shu'bah narrated to us, Mansur ibn al-Mu'tamir informed me, he said: I heard Rib'i ibn Hirash from 'Ali, who said: Among the words most beloved to Allah is that the servant should say: "O Allah, there is no god but You. O Allah, I do not worship anyone except You. O Allah, I do not associate anything with You. O Allah, I have indeed wronged myself, so forgive me my sins, for no one forgives sins except You."