Hadhrat Abu Hurairah (رضى الله تعالى عنه) narrates that Rasulullah (sallallahu صلى الله عليه وسلم) said:
When the Mu’min sins, a black spot forms on his heart. If he makes istighfaar 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 of by Allah Ta’ala (in the Qur’an)
[Ahmad, Tirmizi, Ibn Majah]
Source: Hayatul Muslimeen