“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] (Using translation from Ibn Mājah 4244)
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِنَّ الْمُؤْمِنَ إِذَا أَذْنَبَ ذَنْبًا كَانَتْ نُكْتَةً سَوْدَاءَ فِي قَلْبِهِ فَإِنْ تَابَ وَنَزَعَ وَاسْتَغْفَرَ سُقِلَ مِنْهَا قَلْبُهُ وَإِنْ زَادَ زَادَتْ حَتَّى يَعْلَقَ بِهَا قَلْبُهُ فَذَلِكَ الرَّانُ الَّذِي ذَكَرَ اللَّهُ فِي كِتَابِهِ {كَلَّا بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يَكْسِبُونَ} [المطففين 14]