The Messenger of Allah ﷺ said, "When a believer commits a sin, a black spot appears on his heart. If he repents, gives it up, 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, 'No indeed, rather what they used to earn has rusted upon their hearts' [Mutaffifin 83:14]."
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} [المطففين: ١٤].
Tirmidhi (3334), Ibn Majah (4244), and Ahmad (7952) all narrated 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.Tirmidhi said, "This is a hasan sahih hadith."Hakim said, "This is a sahih hadith according to the criterion of Muslim."