Explanation of the 28 & 29 Hadiths from the Heart Therapy Series:

28) The Prophet salla Allahu alayhi wa sallam said: (When one 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 heart, and this is the Ran which Allah mentioned: ‘No, but on their hearts is the Ran which they earned [al-Mutaffifin 14])
Reported by Ahmad (7952) and al-Tirmidhi (3334) who said it is hasan sahih, and al-Albani declared it to be hasan (Sahih al-Tirmidhi 3334)
29) The Prophet salla Allahu alayhi wa sallam said: (There is at times a thin shade that comes upon my heart, and I ask Allah for forgiveness a hundred times a day)
Reported by Muslim (2702)
قالَ رسولُ اللَّهِ ﷺ: (إِنَّ العَبْدَ إذا أَخْطَأَ خَطيئَةً نُكِتَتْ في قَلْبِهِ نُكْتَةٌ سَوْداءُ فإذا هُوَ نَزَعَ واسْتَغْفَرَ وتابَ سُقِلَ قلبهُ وإن عادَ زِيدَ فيها حتَّى َتعْلُوَ قَلْبَهُ وهو الرَّانُ الذي ذَكَرَ اللهُ كَلّا بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُواْ يَكْسِبُونَ [سورة المطففين 14])
رواه أحمد (7952) والترمذي (3334) وقال حسن صحيح و حسنه الألباني (صحيح الترمذي 3334)
قالَ رسولُ اللَّهِ ﷺ: (إِنَّهُ لَيُغانُ عَلى قَلْبي وإنِّي لأَسْتَغْفِرُ اللهَ في اليَومِ مِائةَ مَرَّةٍ)
رواه مسلم (2702)