Mishkat al-Masabih
Mishkat al-Masabih 2342
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " إِنِ الْمُؤْمِنَ إِذَا أَذْنَبَ كَانَتْ نُكْتَةٌ سَوْدَاءُ فِي قَلْبِهِ فَإِنْ تَابَ وَاسْتَغْفَرَ صُقِلَ قَلْبُهُ وَإِنْ زَادَ زَادَتْ حَتَّى تَعْلُوَ قَلْبَهُ فَذَلِكُمُ الرَّانُ الَّذِي ذَكَرَ اللَّهُ تَعَالَى (كَلَّا بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يكسِبونَ) رَوَاهُ أَحْمَدُ وَالتِّرْمِذِيُّ وَابْنُ مَاجَهْ وَقَالَ التِّرْمِذِيُّ: هَذَا حَدِيثٌ حَسَنٌ صَحِيح
Abu Hurayrah (may God be pleased with him) reported that the Messenger of God (peace be upon him) said: ‘When a believer sins, a black spot appears on his heart. If he repents and asks forgiveness, his heart is polished. If he adds to his sins, the spot increases until it covers his heart. That is the rust God Most High mentioned: “No indeed! Rather, what they used to earn has rusted over their hearts.”’ Ahmad, al-Tirmidhi, and Ibn Majah transmitted it. Al-Tirmidhi said: ‘This is a hasan sahih hadith.’ The Arabic source marks it as not studied by al-Albani.