Allah says:

[فَأَتْبَعَ سَبَباً ]

(So he followed a way.) Ibn `Abbas said that he followed different routes to achieve what he wanted.

[فَأَتْبَعَ سَبَباً ]

(So he followed a way.) Mujahid said that he followed different routes, east and west. According to one report narrated from Mujahid, he said:

[سَبَباً]

(a way) means, "A route through the land.'' Qatadah said, "It means he followed the routes and landmarks of the earth.''

[حَتَّى إِذَا بَلَغَ مَغْرِبَ الشَّمْسِ]

(Until, when he reached the setting place of the sun,) means, he followed a route until he reached the furthest point that could be reached in the direction of the sun's setting, which is the west of the earth. As for the idea of his reaching the place in the sky where the sun sets, this is something impossible, and the tales told by storytellers that he traveled so far to the west that the sun set behind him are not true at all. Most of these stories come from the myths of the People of the Book and the fabrications and lies of their heretics.

[وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍ]

(he found it setting in a spring of Hami'ah) meaning, he saw the sun as if it were setting in the ocean. This is something which everyone who goes to the coast can see: it looks as if the sun is setting into the sea but in fact it never leaves its path in which it is fixed. Hami'ah is, according to one of the two views, derived from the word Hama'ah, which means mud. This is like the Ayah:

[إِنِّى خَـلِقٌ بَشَرًا مِّن صَلْصَـلٍ مِّنْ حَمَإٍ مَّسْنُونٍ]

("I am going to create a man (Adam) from dried clay of altered Hama'h (mud)) [15:28], which means smooth mud, as we have discussed above.

[وَوَجَدَ عِندَهَا قَوْماً]

(And he found near it a people.) meaning a nation. They mentioned that they were a great nation from among the sons of Adam.

[قُلْنَا يذَا الْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْناً]

(We (Allah) said (by inspiration): "O Dhul-Qarnayn! Either you punish them or treat them with kindness'') means, Allah gave him power over them and gave him the choice: if he wanted to, he could kill the men and take the women and children captive, or if he wanted to, he could set them free, with or without a ransom. His justice and faith became apparent in the ruling he pronounced:

[أَمَّا مَن ظَلَمَ]

(As for him who does wrong,) meaning who persists in his Kufr and in associating others in worship with his Lord,

[فَسَوْفَ نُعَذِّبُهُ]

(we shall punish him,) Qatadah said, i.e., by killing him.

[ثُمَّ يُرَدُّ إِلَى رَبِّهِ فَيُعَذِّبُهُ عَذَاباً نُّكْراً]

(and then he will be brought back unto his Lord, Who will punish him with a terrible torment.) meaning a severe, far-reaching and painful punishment. This implies a confirmation of the Hereafter and the reward and punishment.

[وَأَمَّا مَنْ آمَنَ]

(But as for him who believes), meaning `who follows us in our call to worship Allah Alone with no partner or associate,'

[فَلَهُ جَزَآءً الْحُسْنَى]

(he shall have the best reward,) meaning in the Hereafter, with Allah.

[وَسَنَقُولُ لَهُ مِنْ أَمْرِنَا يُسْراً]

(and we (Dhul-Qarnayn) shall speak unto him mild words.) Mujahid said, `(words of) kindness.'

[ثُمَّ أَتْبَعَ سَبَباً - حَتَّى إِذَا بَلَغَ مَطْلِعَ الشَّمْسِ وَجَدَهَا تَطْلُعُ عَلَى قَوْمٍ لَّمْ نَجْعَل لَّهُمْ مِّن دُونِهَا سِتْراً - كَذَلِكَ وَقَدْ أَحَطْنَا بِمَا لَدَيْهِ خُبْراً ]

(89. Then he followed (another) way,) (90. Until, when he came to the rising place of the sun, he found it rising on a people for whom We (Allah) had provided no shelter against the sun.) (91. So (it was)! And We knew all about him (Dhul-Qarnayn).)