Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “Verily, when the servant commits a sin, a black mark appears upon his heart. If he abandons the sin, seeks forgiveness, and repents, then his heart will be polished. If he returns to the sin, the blackness will be increased until it overcomes his heart. It is the covering that Allah has mentioned: No, rather a covering is over their hearts from what they have earned.” (83:14)
Source: Sunan al-Tirmidhī 3334
Grade: Hasan (fair) according to Al-Albani
عَنْ أَبِي هُرَيْرَةَ عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ 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 كَلَّا بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يَكْسِبُونَ
3334 سنن الترمذي كتاب تفسير القرآن باب ومن سورة ويل للمطففين
3141 المحدث الألباني خلاصة حكم المحدث حسن في صحيح الترغيب