The Messenger of Allah ﷺ said, "When a believer commits a sin, a black mark appears on his heart. If he repents, abandons the sin, and seeks forgiveness, his heart is polished clean. But if he persists, the mark 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]."
حدثنا هشام بن عمار حدثنا حاتم بن إسمعيل والوليد بن مسلم قالا حدثنا محمد بن عجلان عن القعقاع بن حكيم عن أبي صالح
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}
[Hasan] Reported by Tirmidhi, tafsir of the Quran, chapter on Surah "Woe to the defrauders", no. 3334, from the hadith of Muhammad bin Ajlan, with this wording, and he said, hasan sahih. Busiri graded it sahih, as did Ibn Hibban, nos. 1771 and 2448, and Hakim graded it as meeting the conditions of Muslim, 2/517, and Dhahabi agreed with him. The hadith has supporting narrations.