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Surah 18. Al-Kahf
Ayah 84 - 88
84 إِنَّا مَكَّنَّا لَهُۥ فِى ٱلْأَرْضِ وَءَاتَيْنَـٰهُ مِن كُلِّ شَىْءٍ سَبَبًا
85 فَأَتْبَعَ سَبَبًا
86 حَتَّىٰٓ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍ وَوَجَدَ عِندَهَا قَوْمًا ۗ قُلْنَا يَـٰذَا ٱلْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْنًا
87 قَالَ أَمَّا مَن ظَلَمَ فَسَوْفَ نُعَذِّبُهُۥ ثُمَّ يُرَدُّ إِلَىٰ رَبِّهِۦ فَيُعَذِّبُهُۥ عَذَابًا نُّكْرًا
88 وَأَمَّا مَنْ ءَامَنَ وَعَمِلَ صَـٰلِحًا فَلَهُۥ جَزَآءً ٱلْحُسْنَىٰ ۖ وَسَنَقُولُ لَهُۥ مِنْ أَمْرِنَا يُسْرًا
[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.”
63"The limit where the sun set" does not mean the "place" of the setting of the sun. According to Ibn Kathir, it means that he marched to the West conquering one country after the other till he reached the last boundary of the land, beyond which there was ocean.
64"He found the sun setting in black muddy waters of the sea": if Zul Qarnain was Cyrus, then that place would be the western limit of Asia Minor and the "black waters" would be the Aegean Sea. This interpretation is supported by the use of the word "`ain"instead of "bahr" in the Qur'an.
65"We said to him" does not necessarily mean that Allah directly revealed to him these words, and that Zul-Qarnain was a Prophet or was the one who received inspiration from Allah, and the same is the reasonable conjecture. This concerns the time when Zul-Qarnain had taken possession of the land as a conqueror and the conquered people were utterly at your mercy. Then Allah posed a question before his conscience, as if to say, "Now is the time of your trial. These people are utterly at your mercy, and you have the option either to behave unjustly towards them or to treat them generously."