It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:“When the believer commits sin, a black spot appears on his heart. If he repents and gives up that sin and seeks forgiveness, his heart will be polished. But if (the sin) increases, (the black spot) increases. That is the Ran that Allah mentions in His Book: “Nay! But on their hearts is the Ran (covering of sins and evil deeds) which they used to earn.” [83:]

English translation · Sunan Ibn Majah