Musnad Ahmad 5:213

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