Hazrat Muhammad (SAW) said “When a person commits a sin, a black dot is dotted on his heart. Then if that person gives up that sin, begs Allah to forgive him and repents, then his heart is cleared (from that covering dot); but if he repeats the sin, then that covering is increased till his heart is completely covered with it. And that is Ar-Ran which Allah has mentioned in the Quran (Surah 82, ayat no 14)”.
Jami` at-Tirmidhi Vol 6, Hadith No. 3334