Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Just as a cloud comes before the moon (and dims or blocks its light), so too does a cloud of (sins) cover every heart. (You see that) the moon is shining, then suddenly a cloud comes before it and it becomes dark, and when the cloud moves away, it becomes bright again (the same is the case with the heart)."