Sayyidina Abu Hurayrah (RA) reported that Allah's Messenger (SAW) said; "When someone commits a sin, a black dot is marked on his heart. When he abandons it and seeks forgiveness and repents, his heart is cleaned (and spotless), but if he persists and returns (to the sin), then the dots are added till blackness covers his heart as Allah has said: "By no means! but on their hearts is the stain of the (ill) which they do! (83; 14)
[Ahmed 7957, Muslim 4244, Nisai 418]