Muhammad - 47:16
"There are some of them who listen to you. But when they leave you, they ask those to whom they have been given knowledge, "What did he say just now?"These are they whose hearts Allah has stamped, and they follow their desires."
That their understanding may be completely removed, that they may never again find the right path:
The Women/4:155
"Allah sealed their understanding because of their breaking their word, because they disbelieved in the signs and proofs of Allah, because they killed the prophets unjustly, and because they said: "We are fixed-minded", yes, because of their disbelief. Therefore few of them believe."
The Kneeling/45:23
Have you paid attention to him who takes his desire as a god? Allah has deliberately led him astray, and has sealed his hearing and his brain and put a veil over his sight. Who can guide him to the truth except Allah? Do you not take admonition?
The Hypocrites/63:3
Because they disbelieved after they believed, so their hearts were sealed; they do not understand.
The Pilgrimage/22:46
Have they not traveled the earth with thinking minds and hearing ears? The real blindness is not the blindness of the eyes, but the blindness of the hearts in the breasts.
The Heights/7:100
Has it not become evident to those who inherited the earth in place of the previous generations that if We willed, We could punish them with their sins and seal their hearts so that they could not hear?