رَانَ (stain): It is the rust on the hearts. Guilts and sins gathered on the hearts and blinded it and on the visions and wiped them, so, they do not recognize what is good or reject what is abominable.
The Prophet (saws) described this case in some hadiths, Saydna Hudhayfa Ibn Al-Yaman (May Allah be pleased with him) said: I heard the Messenger of Allah (saws) saying:
“Temptations will be presented to men’s hearts as reed mat is woven stick by stick and any heart which is impregnated by them will have a black mark put into it, but any heart which rejects them will have a white mark put in it. The result is that there will become two types of hearts: one white like a white stone which will not be harmed by any turmoil or temptation, so long as the heavens and the earth endure; and the other black and dust-coloured like a vessel which is upset, not recognizing what is good or rejecting what is abominable, but being impregnated with passion.”
 Sahih Muslim: Book of Faith, Hadith No. 144