In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Allah, The Most High, said:

وَلَا تُفْسِدُوا فِي الْأَرْضِ بَعْدَ إِصْلَاحِهَا وَادْعُوهُ خَوْفًا وَطَمَعًا ۚ إِنَّ رَحْمَتَ اللَّهِ قَرِيبٌ مِّنَ الْمُحْسِنِينَ

And do not do mischief on the earth, after it has been set in order, and invoke Him with fear and hope; Surely, Allah’s Mercy is (ever) near unto the good-doers. [Al-A’raf 56]

“And do not do mischief on the earth, after it has been set in order”.

That is: do not spread corruption therein through acts of disobedience and calling to other than obedience to Allah, after Allah has set it in order by sending the Messengers, clarifying the divine law, and calling (humankind and jinn) to obey Allah.

This meaning is the statement of al-Hasan, As-Suddiy, Ad-Dahhak, and al-Kalbi. [1] 

Allah, The Most High, said:

قَالُوا يَا ذَا الْقَرْنَيْنِ إِنَّ يَأْجُوجَ وَمَأْجُوجَ مُفْسِدُونَ فِي الْأَرْضِ

 فَهَلْ نَجْعَلُ لَكَ خَرْجًا عَلَىٰ أَن تَجْعَلَ بَيْنَنَا وَبَيْنَهُمْ سَدًّا

قَالَ مَا مَكَّنِّى فِيهِ رَبِّى خَيْرٌ

فَأَعِينُونِى بِقُوَّةٍ أَجْعَلْ بَيْنَكُمْ وَبَيْنَهُمْ رَدْمًا

ءَاتُونِى زُبَرَ ٱلْحَدِيدِ حَتَّىٰٓ إِذَا سَاوَىٰ بَيْنَ ٱلصَّدَفَيْنِ

قَالَ ٱنفُخُوا۟ حَتَّىٰٓ إِذَا جَعَلَهُۥ نَارًا قَالَ ءَاتُونِىٓ أُفْرِغْ عَلَيْهِ قِطْرًا

فَمَا ٱسْطَٰعُوٓا۟ أَن يَظْهَرُوهُ وَمَا ٱسْتَطَٰعُوا۟ لَهُۥ نَقْبًا

قَالَ هَٰذَا رَحْمَةٌ مِّن رَّبِّى

فَإِذَا جَآءَ وَعْدُ رَبِّى جَعَلَهُۥ دَكَّآءَ وَكَانَ وَعْدُ رَبِّى حَقًّا

They said: “O Dhul-Qarnain! Verily! Ya’juj and Ma’juj (Gog and Magog) are doing great mischief in the land. Shall we then pay you a tribute in order that you might erect a barrier between us and them?”

He said: “That (wealth, authority and power) in which my Lord had established me is better (than your tribute). So help me with strength (of men), I will erect between you and them a barrier.

“Give me pieces (blocks) of iron,” then, when he had filled up the gap between the two mountain-cliffs, he said: “Blow,” till when he had made it (red as) fire, he said: “Bring me molten copper to pour over it.”

So they [Ya’juj and Ma’juj (Gog and Magog)] were made powerless to scale it or dig through it.

Dhul-Qarnain) said: “This is a mercy from my Lord, but when the Promise of my Lord comes, He shall level it down to the ground. And the Promise of my Lord is ever true.” [Al-Kahf 94-98]

إِنَّ يَأْجُوجَ وَمَأْجُوجَ مُفْسِدُونَ فِي الْأَرْضِ

“Verily! Yajuj and Majuj are doing great mischief in the land” – through murder, seizing wealth, and other than that.

“Shall we pay you a tribute” — meaning a payment— “in order that you might erect a barrier between us and them?”

This shows that they were unable to erect the barrier themselves, while they knew Dhul-Qarnain’s ability to do so. Thus, they offered him a payment and mentioned to him the reason for such (request), and that was due to (Yajuj and Majuj) spreading corruption in the land.

But neither was Dhul-Qarnayn one who was covetous, nor desirous of worldly things, nor did he abandon the welfare of the subjects, rather, his intention was rectificatiom. Due to this, he responded to their request because of the welfare therein, and he did not take payment from them, and he thanked his Lord for granting him the empowerment and enablement (to fulfil the task). [2]

[1] Paraphrased

ولا تفسدوا في الأرض بعد إصلاحها ) أي لا تفسدوا فيها بالمعاصي والدعاء إلى غير طاعة الله بعد إصلاح الله إياها ببعث الرسل وبيان الشريعة ، والدعاء إلى طاعة الله ، وهذا معنى قول الحسن والسدي والضحاك والكلبي

Tafsir Al-Baghawi

[2] Paraphrased

إِنَّ يَأْجُوجَ وَمَأْجُوجَ مُفْسِدُونَ فِي الْأَرْضِ ْ

 بالقتل وأخذ الأموال وغير ذلك.

{ فَهَلْ نَجْعَلُ لَكَ خَرْجًا ْ} أي جعلا { عَلَى أَنْ تَجْعَلَ بَيْنَنَا وَبَيْنَهُمْ سَدًّا ْ} ودل ذلك على عدم اقتدارهم بأنفسهم على بنيان السد، وعرفوا اقتدار ذي القرنين عليه، فبذلوا له أجرة، ليفعل ذلك، وذكروا له السبب الداعي، وهو: إفسادهم في الأرض، فلم يكن ذو القرنين ذا طمع، ولا رغبة في الدنيا، ولا تاركا لإصلاح أحوال الرعية، بل كان قصده الإصلاح، فلذلك أجاب طلبتهم لما فيها من المصلحة، ولم يأخذ منهم أجرة، وشكر ربه على تمكينه واقتداره

Tafsir As-Sadi