Musnad Abu Ya'la al-Mawsili 1:15

Ubayd Allah ibn Umar al-Jushami narrated to us, Yahya ibn Sa’id narrated to us, Sufyan ibn Sa’id al-Thawri narrated to us, Uthman ibn al-Mughirah al-Thaqafi narrated to us. He said: And Ubayd Allah ibn Umar narrated to us, Muhammad ibn Abd Allah ibn al-Zubayr narrated to us, Sufyan narrated to us, from Uthman ibn al-Mughirah, from Ali ibn Rabi’ah, from Asma ibn al-Hakam al-Fazari, from Ali, who said: "When I was told a hadith from the Messenger of Allah (may Allah bless him and grant him peace), I would make its narrator swear an oath, and if he swore to me, I would believe him. Abu Bakr told me—and Abu Bakr spoke the truth—that he (may Allah bless him and grant him peace) said: 'There is no servant who commits a sin, then performs ablution, prays two rak'ahs, and asks Allah for forgiveness, but that He forgives him.'"