Quran 18:87 — The Cave (Al-Kahf)

قالَ أَمّا مَن ظَلَمَ فَسَوفَ نُعَذِّبُهُ ثُمَّ يُرَدُّ إِلىٰ رَبِّهِ فَيُعَذِّبُهُ عَذابًا نُكرًا

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.

[18:86] 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."

[18:88] "As for those who believe and lead a righteous life, they receive a good reward; we will treat them kindly."