Musnad Ahmad 1:46

'Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from 'Uthman ibn al-Mughirah, who said: I heard 'Ali ibn Rabi'ah, from the tribe of Banu Asad, narrating from Asma' or the son of Asma' from the tribe of Banu Fazarah, who said: 'Ali (may Allah be pleased with him) said: When I heard something from the Messenger of Allah (peace and blessings of Allah be upon him), Allah would benefit me with it as much as He willed to benefit me. And Abu Bakr narrated to me, and Abu Bakr spoke the truth. He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no Muslim who commits a sin, then performs ablution, then prays two rak'ahs, then seeks Allah's forgiveness for that sin, but He will forgive him." And he recited these two verses: "And whoever does evil or wrongs himself but then seeks forgiveness of Allah will find Allah Forgiving and Merciful" [An-Nisa': 110], and "And those who, when they commit an immorality or wrong themselves [by transgression]..." [Al 'Imran: 135].