Musnad Ahmad 5:795

Hajjaj narrated to us, he said: Yunus bin Abi Ishaq informed me, from Abu Ishaq, from Abu Juhayfah, from Ali, may Allah be pleased with him, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever commits a sin in the world and is punished for it, then Allah is more just than to repeat the punishment upon His servant. And whoever commits a sin in the world and Allah conceals it for him and pardons him, then Allah is more generous than to return to something He has already pardoned."