The Messenger of Allah ﷺ said, "When a believer commits a sin, a black spot appears upon his heart. If he repents, abandons the sin, and seeks Allah's forgiveness, his heart is polished clean. But if he sins more, the spot grows larger. That is the rust which Allah mentioned in His Book, 'No! Rather, what they have been doing has cast a covering over their hearts' [Mutaffifin 83:14]."
أن رسول الله ﷺ قال: إن المؤمن إذا أذنب كانت نكتة سوداء في قلبه فإن تاب ونزع واستغفر الله صقل قلبه، فإن زاد زادت، فذلك الران الذي ذكره الله في كتابه: {كلا بل ران على قلوبهم ما كانوا يكسبون} [ المطففين: ١٤].
Tirmidhi (3334), Ibn Majah (4244), and Ahmad (7952) reported it, and Ibn Hibban (930) and Hakim (2/517) graded it sahih, all of them from the hadith of Muhammad bin Ajlan, from Qaqa bin Hakeem, from Abu Salih, from Abu Hurairah, who mentioned it. Its chain is hasan because of Muhammad bin Ajlan, for he is hasan al-hadith. The paths of knowledge were closed off to them because of their disbelief and their denial after the truth had become clear to them.