Abu Hurairah narrated that :
the Messenger of Allah said: "Verily, when the slave (of Allah) commits a sin, a black spot appears on his heart. When he refrains from it, seeks forgiveness and repents, his heart is polished clean. But if he returns, it increases until it covers his entire heart. And that is the 'Ran' which Allah mentioned: 'Nay, but on their hearts is the Ran which they used to earn.'"
English reference: Vol. 5, Book 44, Hadith 3334
Arabic reference: Book 47, Hadith 3654