Musnad Ahmad 30:834

Safwan ibn 'Isa narrated to us, Muhammad ibn 'Ajlan informed us, from al-Qa'qa' ibn Hakim, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, when the believer commits a sin, there is a black dot in his heart. If he repents, desists, and seeks forgiveness, his heart is polished. But if he increases [in sin], it increases, until that *ran* (covering/rust) covers his heart, which Allah the Almighty mentioned in the Qur'an: 'Nay, but that which they have earned has covered their hearts' [Al-Mutaffifin: 14]."