﴿ حَتَّىٰٓ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍۢ وَوَجَدَ عِندَهَا قَوْمًۭا ۗ قُلْنَا يَٰذَا ٱلْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْنًۭا ﴾

“[And he marched westwards] till, when he came to the setting of the sun, it appeared to him that it was setting in a dark, turbid sea; and nearby he found a people [given to every kind of wrongdoing]. We said: "O thou Two-Horned One! Thou mayest either cause [them] to suffer or treat them with kindness!"”

Please check ayah 18:88 for complete tafsir.