Sin and seek forgiveness

Prophet (PBUH) Said: "When the servant performs a sin, a black spot appears on his heart, & if he seeks forgiveness this black spot is removed, & if he returns to sin the black spot grows until his heart becomes black, and this is the 'Raan' about which Allaah spoke, "Nay! But on their hearts is the covering of sins (raan) which they used to earn."
[(83):14] [Nasaa'ee and at-Tirmidhee; hasan saheeh.]