"When a servant commits a sin, a black spot appears on his heart. If he repents, it is removed. If he increases in sin, it increases until it covers his heart — that is the 'rān' (covering) which Allah mentioned: 'Nay! Rather their hearts have been covered by what they have earned.'"
Jami' at-Tirmidhi 3334 — Narrated by Abu Huraira (RA); referencing Surah Al-Mutaffifin 83:14