It wasnarrated from Abu Hurairah that the Messenger of Allah (ﷺ)said:
"When the believer commits sin, a black spot appears on hisheart.If he repents and gives up that sin and seeks forgiveness, hisheartwill be polished. But if (the sin) increases, (the black spot)increases. That is the Ran that Allah mentions in His Book: "Nay!Buton their hearts is the Ran (covering of sins and evil deeds)whichthey used to earn." [83:14]
English reference: Vol. 5, Book 37, Hadith 4244
Arabic reference: Book 37, Hadith 4385