483.Hadhrat Abu Hurairah (Allah be pleased with him) narrates that Muhammad (peace be upon him) said: "When a Mum'in sins, a black spot forms on his heart. If he makes isitighfaar and taubah, the black mark is removed and his heart is purified. If he increases his sin, the black mark increases. This is the corrosion of the heart spoken by Allah Ta'ala (in the Qur'an)."
(Ahmad, Tirmidhi, Ibn Majah).
(Hayaat-ul-Muslimeen. by Mohammad Ashraf Ali Thanvi).