It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, when a believer commits a sin, a black spot appears on his heart. Then if he repents, desists (from the sin) and seeks forgiveness, his heart is polished clean; but if he increases (in sin), the blackness increases until it covers his whole heart. And that is the 'ran' (rust) which Allah Almighty has mentioned in His Book: 'No! Rather, the rust of what they used to earn has covered their hearts.' [Surah al-Mutaffifin: 14] " This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.