As requested
http://www.englishtafsir.com/Quran/18/index.html#sdfootnote65sym
[84-88] We had established his power on the Earth and had provided him with every kind of ways and means. At first, he made preparations for an expedition (to the West and marched on) till he reached the limit where the sun set,
63 and found it setting in black waters,
64 and there he saw a people. We said to him, “O Zul-Qarnain! You have the power to punish them and also the option to treat them generously.”
65 He said, “We will punish that one of them who will commit iniquity: then he shall be returned to his Lord and He will inflict on him a grievous torment: And as for the one, who will believe and do righteous deeds, there is a generous recompense and We will prescribe for him easy tasks.”
[89-91] Then he made preparations (for another expedition and marched on) till he reached the limit where the sun rose. There he saw the sun rising on a people, whom We had not given any shelter from sun shine.
66 This was their condition and We know well whatever Zul-Qarnain possessed.
tafseer of Jalalayn
"Until he reached the setting place of the sun" "He found it setting in a muddy spring"