Musnad Ahmad 1:55

Abu Kamil narrated to us, Abu 'Awanah narrated to us, 'Uthman ibn Abi Zur'ah narrated to us, from 'Ali ibn Rabi'ah, from Asma' ibn al-Hakam al-Fazari, who said: I heard 'Ali say: When I heard a hadith from the Messenger of Allah, may Allah bless him and grant him peace, Allah would benefit me with it as much as He willed to benefit me. And when someone else narrated to me, I would make him swear an oath; if he swore to me, I would believe him. Abu Bakr narrated to me, and Abu Bakr spoke the truth. He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "There is no believing servant who commits a sin, then performs ablution and perfects the purification, then prays two rak'ahs and seeks the forgiveness of Allah, but that Allah forgives him." Then he recited: {And those who, when they commit an immorality or wrong themselves [by transgression]} [Al 'Imran: 135].