The word(s) "walls" appears 3 time(s) in 3 verse(s) in Quran in Saheeh International translation.

(1) And say, "The truth is from your Lord, so whoever wills - let him believe; and whoever wills - let him disbelieve." Indeed, We have prepared for the wrongdoers a fire whose walls will surround them. And if they call for relief, they will be relieved with water like murky oil, which scalds [their] faces. Wretched is the drink, and evil is the resting place.

(2) Bring me sheets of iron" - until, when he had leveled [them] between the two mountain walls, he said, "Blow [with bellows]," until when he had made it [like] fire, he said, "Bring me, that I may pour over it molten copper."

(3) They will not fight you all except within fortified cities or from behind walls. Their violence among themselves is severe. You think they are together, but their hearts are diverse. That is because they are a people who do not reason.