Quran Al-Kahf 18:86

Translation

Until, when he reached the setting of the sun [i.e., the west], he found it [as if] setting in a body of dark water, and he found near it a people. We [i.e., Allāh] said, "O Dhul-Qarnayn, either you punish [them] or else adopt among them [a way of] goodness."