Mishkat al-Masabih #2240

Abu Huraira reported God’s messenger as saying, “When a believer sins there is black spot in his heart, and if he repents and asks pardon his heart is polished; but if he does more it increases till it gains the ascendancy over his heart. That is the rust mentioned by God most high, 'Nay, but what they were committing has spread like rust over their hearts'.” 1 1. Qur’an, lxxxiii, 14. Ahmad, Tirmidhi and Ibn Majah transmitted it, Tirmidhi saying this is a hasan sahih tradition.