Surat Al-Kahf (The Cave) - الكهف

حَتَّىٰٓ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍ وَوَجَدَ عِندَهَا قَوْمًا قُلْنَا يَٰذَا ٱلْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْنًا ﴿٨٦﴾

Until when he reached the place where the sun set, he found it going down into a black sea, and found by it a people. We said: O Zulqarnain! either give them a chastisement or do them a benefit.