It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: When a sin is committed by a Muslim, a black spot appears on his heart. If he repents and seeks forgiveness, his heart becomes clean and bright again. Otherwise, as the sins increase, so do the black spots, until rust overtakes his heart, as Allah has mentioned in the Noble Qur’an with these words: "Nay! But on their hearts is the rust of that which they used to earn."