There is no servant except that there exists a white spot upon his heart. So when he commits a sin, a black spot arises into that white spot. Then when he repents, this black spot moves away. But if he continues on committing sins, the blackness of this spot increases till such time that it overwhelms and overrides the whiteness. When the whiteness is all covered over (by the blackness), the owner of it (the heart) does never at all return towards beneficence and goodness. And This is what Allah means when he says: "Nay! rather, what they used to do has become like a rust upon their hearts."