Quran 18:86 — The Cave (Al-Kahf)

When he reached the far west, he found the sun setting in a vast ocean, and found people there. We said, "O Zul-Qarnain, you can rule as you wish; either punish, or be kind to them."

[18:85] Then, he pursued one way.

[18:87] He said, "As for those who transgress, we will punish them, then, when they return to their Lord, He will commit them to more retribution.