It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said: “When the believer commits sin, a black spot appears on his heart. If he repents and gives up that sin and seeks forgiveness, his heart will be polished. But if (the sin) increases, (the black spot) increases. That is the Ran that Allah mentions in His Book: “Nay! But on their hearts is the Ran (covering of sins and evil deeds) which they used to earn.” [83:14]
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا حَاتِمُ بْنُ إِسْمَاعِيلَ، وَالْوَلِيدُ بْنُ مُسْلِمٍ، قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ عَجْلاَنَ، عَنِ الْقَعْقَاعِ بْنِ حَكِيمٍ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ قَالَ : " إِنَّ الْمُؤْمِنَ إِذَا أَذْنَبَ كَانَتْ نُكْتَةٌ سَوْدَاءُ فِي قَلْبِهِ فَإِنْ تَابَ وَنَزَعَ وَاسْتَغْفَرَ صُقِلَ قَلْبُهُ فَإِنْ زَادَ زَادَتْ فَذَلِكَ الرَّانُ الَّذِي ذَكَرَهُ اللَّهُ فِي كِتَابِهِ {كَلاَّ بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يَكْسِبُونَ} " .
sunnah.com