Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When a servant commits a sin, a black spot is placed on his heart. If he repents, abandons the sin and seeks forgiveness, the spot is wiped clean. If he sins again, the heart becomes covered with rust." Allah has mentioned this in His Book: "By no means! Rather, their hearts have become rusted because of what they used to earn" [Surah al-Mutaffifin: 14].
The Shaykh said: This is a rebuke for those who persist in sins, who do not repent and seek forgiveness, and who do not resolve in their hearts to give up the sin.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20763