Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

that he heard Sayyidina Ali (RA) say, "When I heard something from Allah's Messenger (SAW) Allah gave me benefit from it as much as He willed. And when one of the sahabah narrated something to me, I made him say that on oath. So, when he took the oath for me, I would confirm him. And, indeed, Abu Bakr (RA) narrated to me, and he spoke the truth, that he heard Allah's Messenger say: There is not a man who sins then stands up, purifies himself and offers salah, and then seeks Allah's forgiveness but Allah forgives him. Then he recited the verse "And those who when they commit an indecency or do injustice to their souls remember Allah and ask forgiveness for their faults - and who forgives the faults but Allah" to the end of it."