(Al-Kahf) 18 : 86
حَتَّىٰ إِذَا بَلَغَ مَغْرِبَ الشَّمْسِ وَجَدَهَا تَغْرُبُ فِي عَيْنٍ حَمِئَةٍ وَوَجَدَ عِنْدَهَا قَوْمًا ۗ قُلْنَا يَا ذَا الْقَرْنَيْنِ إِمَّا أَنْ تُعَذِّبَ وَإِمَّا أَنْ تَتَّخِذَ فِيهِمْ حُسْنًا
Until when he reached the setting of the sun, he found it setting in a muddy lake, and he found near it a people; Lord Allah said: "Oh Dhul-Qarnayn, either you can punish them or you can adopt them in goodness".
The sun setting place means the utmost west region where the human inhabitance existed at that time.