Abu Hurairah reported,

The Messenger of Allah ﷺ said, "When a believer commits a sin, a black spot appears on his heart. If he repents, abandons the sin, and seeks forgiveness, his heart is polished clean. But if he persists, the spot grows. That is the rust which Allah mentioned in His Book, 'Nay, but on their hearts is the stain of what they used to earn' [Mutaffifin 83:14]."

عن أبي هريرة

أن رسول الله ﷺ قال: إن المؤمن إذا أذنب كانت نكتة سوداء في قلبه. فإن تاب ونزع واستغفر صقل قلبه. فإن زاد زادت. فذلك الران الذي ذكره الله في كتابه: {كلا بل ران على قلوبهم ما كانوا يكسبون}