Allāh—the Most High—said:

وَلَا تَمُدَّنَّ عَيْنَيْكَ إِلَىٰ مَا مَتَّعْنَا بِهِ أَزْوَاجًا مِّنْهُمْ زَهْرَةَ الْحَيَاةِ الدُّنْيَا لِنَفْتِنَهُمْ فِيهِ ۚ وَرِزْقُ رَبِّكَ خَيْرٌ وَأَبْقَىٰ

“And strain not your eyes in longing for the things We have given for enjoyment to various groups of them (polytheists and disbelievers in the Oneness of Allāh), the splendour of the life of this world that We may test them thereby. But the provision (good reward in the Hereafter) of your Lord is better and more lasting.”
(Ṭā-Há, 20:131)

Imām ʿAbd al-Raḥmān ibn Nāṣir al-Saʿdī comments:

This verse encourages and stipulates asceticism¹ of this worldly life. That the opulent, luxurious, comfortable lives wrought with goodness and excess which the rich enjoy is completely unrelated to any magnanimous station occupied by them with Allāh. Rather, what they have been granted is only a trial for them, meant as a test. Such that it may be used to determine the best of them in conduct, the most complete and perfect in intellectual faculties. For indeed, the truly intellectual one pays more care and attention to that which is both precious and long-lasting over lowly, base, transient pleasures. For this reason, He said: “But the provision (good reward in the Hereafter) of your Lord…” that is, what He has prepared for the obedient ones who have opted out from joining the rich in their extravagant, luxurious, excessive lives. This is because they have not been deceived by the brilliant radiance of this worldly life and the transient enjoyment therein. Instead, they have chosen to gaze upon the inner, inapparent meaning of these things while the ignorant ones can only observe its outward, apparent form. The obedient ones are able to truly acknowledge what is intended by these things, fully able to discern the disparity and levels of this matter between it [and the enjoyment of the hereafter]. Thus, the provision of Allāh is better and more lasting for these people specifically. That is, it represents a higher degree, in every conceivable measure, of perfection and completion. In addition, it is eternal and long lasting, never leaving them.

As for the pleasures enjoyed by the people of this worldly life, it is all representative of the “splendours of the worldly life”. It comes quickly and shall all fade away. For this reason, Allāh has forbade His Messenger (صلى الله عليه وسلم) from straining his eyes in longing for the things they enjoy. Straining one’s eyes: that is, seeking to be acquainted with and honourably bestowed with it. It is not just to simply gaze upon what they have amassed. Rather, it refers to gazing with one’s heart. As He does not say: And do not look at the things We have given them for enjoyment; rather, straining one’s eyes [as in the verse] stipulates commendation with one’s heart and seeking acquaintance with their possessions in relation to it. Likewise, the verse:

وَاصْبِرْ نَفْسَكَ مَعَ الَّذِينَ يَدْعُونَ رَبَّهُم بِالْغَدَاةِ وَالْعَشِيِّ يُرِيدُونَ وَجْهَهُ ۖ وَلَا تَعْدُ عَيْنَاكَ عَنْهُمْ تُرِيدُ زِينَةَ الْحَيَاةِ الدُّنْيَا

“And keep yourself (O Muḥammad (صلى الله عليه وسلم)) patiently with those who call on their Lord (i.e. your companions who remember their Lord with glorification, praising in prayers, etc., and other righteous deeds, etc.) morning and afternoon, seeking His Face, and let not your eyes overlook them, desiring the pomp and glitter of the life of the world.”
(Al-Kahf, 18:28)

This verse clarifies what is meant from the aforementioned one which is that gazing with the eyes here is in attachment to a desire that is harboured for the pomp and glitter of this worldly life. The same sentiment is expressed in the saying of the Most High:

‏‏ وَلَقَدْ آتَيْنَاكَ سَبْعًا مِّنَ الْمَثَانِي وَالْقُرْآنَ الْعَظِيمَ ‎﴿٨٧﴾‏ لَا تَمُدَّنَّ عَيْنَيْكَ إِلَىٰ مَا مَتَّعْنَا بِهِ أَزْوَاجًا مِّنْهُمْ وَلَا تَحْزَنْ عَلَيْهِمْ وَاخْفِضْ جَنَاحَكَ لِلْمُؤْمِنِينَ

“And indeed, We have bestowed upon you seven of Al-Mathānī (the seven repeatedly recited verses), (i.e. Sūrat al-Fātiḥah) and the Grand Qurʾān. Look not with your eyes ambitiously at what We have bestowed on certain classes of them (the disbelievers), nor grieve over them. And lower your wings for the believers (be courteous to the fellow-believers).”
(Al-Ḥijr, 15:88)

Here, he (صلى الله عليه وسلم) is encouraged to, instead, engage in the exultation and triumph of what he was given by Allāh from al-Mathānī [Surah al-Fātiḥah] and the grand Qurʾān, to remember that which he (صلى الله عليه وسلم) has been blessed with. For it is truly representative of goodness, blessings, and mercy which is more deserving of happiness and rejoicement. As it is far superior to what the people of this worldly life have amassed and find enjoyment and contentment in. Thus, those who observe with true exultation and triumph are the believers who do not allow themselves to be deceived by that which the opposers to the truth have been deceived by. Due to this, He said: “And lower your wings for the believers (be courteous to the fellow-believers).”

Endnotes:
1. Translator note: For more detail, see Understanding Zuhd (Asceticism) in Islām by Imām Muḥammad ibn Ṣāliḥ al-ʿUthaymin.

Source: Al-Mawāhib al-Rabbāniyyah: 23-24
Translated by: Riyāḍ al-Kanadī