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.

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said concerning the saying of Allah Almighty "Its seal is musk" [Surah al-Mutaffifin: 26] : (What is meant is) a blending of fragrance (in the drink of Paradise), not a seal that is set upon (a vessel) to close it. This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.