The Messenger of Allaah sallAllaahu 'alayhi wa sallam said:

“If the believer commits a sin, a black spot appears on his heart. If he repents, gives it up and seeks forgiveness, his heart is cleansed, but if he does more then (that spot) increases until it covers his heart. That is the raan (covering of sin) which Allaah mentioned in the Qur’aan: “Nay! But on their hearts is the Raan (covering of sins and evil deeds) which they used to earn” [al-Mutaffifeen 83:14]

[Tirmidhi 3334, Hasan by Albani]