Sura al-Kahf 18:86

حَتَّىٰٓ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍ وَوَجَدَ عِندَهَا قَوْمًا قُلْنَا يَٰذَا ٱلْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْنًا

ḥatta idhā balagha maghriba l-shamsi wajadahā taghrubu fī ʿaynin ḥamiʾatin wa-wajada ʿindahā qawman qulnā yādhā l-qarnayni immā an tuʿadhdhiba waimmā an tattakhidha fīhim ḥusnan

Until, when he reached the setting of the sun, he found it set in a spring of murky water: Near it he found a People: We said: "O Zul-qarnain! (thou hast authority,) either to punish them, or to treat them with kindness."