Syedna Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: When a believer commits a sin, a black spot appears on his heart. If he repents and seeks forgiveness, his heart becomes bright and polished. But if he continues to sin, the spots increase until his heart is completely covered with rust, as Allah Almighty mentioned in this verse: Never! Rather, the rust has covered their hearts because of what they used to earn.