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 until it covers his entire heart. That is the rust which Allah mentioned in the Quran, 'No! Rather, the stain has covered their hearts of that which they were earning'[Mutaffifin 83:14]."
حدثنا صفوان بن عيسى أخبرنا محمد بن عجلان عن القعقاع بن حكيم عن أبي صالح
قال رسول الله ﷺ إن المؤمن إذا أذنب كانت نكتة سوداء في قلبه فإن تاب ونزع واستغفر صقل قلبه وإن زاد زادت حتى يعلو قلبه ذاك الرين الذي ذكر الله عز وجل في القرآن {كلا بل ران على قلوبهم ما كانوا يكسبون}
Its isnad is sahih. Tabari also related it in the tafsir, number 304 in our numbering, from Muhammad bin Bashshar, from Safwan bin Isa, with this same isnad, and he related it again [volume 30, page 62 of the Bulaq edition] with this very same isnad. Tirmidhi related it, 4:210, from Qutayba, from Layth, from Ibn Ajlan, with the same wording, and he said, "This is a hasan sahih hadith." Ibn Majah related it, 4244, through the narration of Hatim bin Ismail and Walid bin Muslim, both from Ibn Ajlan. Hakim related it in Mustadrak, 2:517, by way of Bakkar bin Qutayba the judge, from Safwan bin Isa, with the same wording, and he said, "This is a hadith that is sahih according to the criterion of Muslim, though the two of them did not include it." Dhahabi agreed with him.Ibn Kathir mentioned it in the tafsir, 1:84, through the narration of Tabari, and attributed it to Tirmidhi, Nasai, and Ibn Majah, and he mentioned it again at 9:143 through the narration of these same figures, and through the narration of the Musnad. Suyuti mentioned it in Durr al-Manthur, 6:325, and added its attribution to Abd bin Humayd, Ibn Hibban, Ibn Munzir, Ibn Mardawayh, and Bayhaqi in Shuab al-Iman.As for his saying "ar-rayn," in one copy in the margin of M it appears as "ar-ran," and likewise in some of the narrations of those we have mentioned. Both are sahih, ar-rayn and ar-ran being equivalent, like "az-zaym" and "az-zam," or "al-ayb" and "al-aab." The root meaning of ar-rayn is a covering or a defiling stain, and it also refers to the rust that forms over a sword or a mirror. Abu Ubayd said, "Whatever overcomes you and dominates you, it has 'ran' upon you, and 'ranaka,' and 'ran alayka.'"