Jami` at-Tirmidhi, Book 47, Hadith 3654 (In-Book) ⓘ

Abu Hurairah narrated that : the Messenger of Allah said: “Verily, when the slave (of Allah) commits a sin, a black spot appears on his heart. When he refrains from it, seeks forgiveness and repents, his heart is polished clean. But if he returns, it increases until it covers his entire heart. And that is the ‘Ran’ which Allah mentioned: ‘Nay, but on their hearts is the Ran which they used to earn.’”

حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ عَجْلاَنَ، عَنِ الْقَعْقَاعِ بْنِ حَكِيمٍ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ إِنَّ الْعَبْدَ إِذَا أَخْطَأَ خَطِيئَةً نُكِتَتْ فِي قَلْبِهِ نُكْتَةٌ سَوْدَاءُ فَإِذَا هُوَ نَزَعَ وَاسْتَغْفَرَ وَتَابَ سُقِلَ قَلْبُهُ وَإِنْ عَادَ زِيدَ فِيهَا حَتَّى تَعْلُوَ قَلْبَهُ وَهُوَ الرَّانُ الَّذِي ذَكَرَ اللَّهُ ‏:‏ ‏(‏ كلاَّ بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يَكْسِبُونَ ‏)‏ ‏"‏ ‏.‏ قَالَ هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏