al-Moulakhas al-Fiqhi - Salih Al-Fawzan - (2/328)

Then he, peace and blessings be upon him, said: “And whoever is unable”—that is, whoever lacks the capacity to marry and bear its costs—
“must fast,” meaning he should adopt fasting as an alternative remedy.
“For it will be a wijāʾ for him,” that is, it drives the craving away and shields him from its dangers just as wijāʾ—meaning castration—cuts it off altogether. By reducing his intake of food and drink, the fasting person’s passion is subdued, and he experiences a distinctive spiritual state marked by fear of Allah and mindfulness of Him, as the Exalted says: “O you who believe, fasting has been prescribed for you as it was prescribed for those before you so that you may become God-fearing” (Q 2:183); and He says: “And to fast is better for you, if you but knew” (Q 2:184).

Thus the Prophet, peace and blessings be upon him, ordered that desire be resisted and its danger averted through two successive measures:

Marriage whenever one is able to undertake it.
Fasting for the one who cannot marry.
This shows that a person must not leave himself exposed to the pathways of danger.

The same meaning appears in His saying, the Exalted: “Marry off the single among you and the righteous among your male and female slaves…” up to His words: “And let those who find no means to marry keep themselves chaste until Allah enriches them out of His bounty” (Q 24:32–33).

ثم قال ﷺ: "ومن لم يستطع" أي: لا يقدر على النكاح ومؤنه. "فعليه بالصوم" أي: يتخذ الصوم علاجا بديلاً. "فإنه له وجاء" أي: يدفع الشهوة ويجنبه خطرها كما يقطعها الوجاء، وهو الاختصاء؛ لأن الصائم بتقليل الطعام والشراب يحصل له انكسار الشهوة، ويحصل له شعور خاص في حال الصيام من خشية الله وتقواه؛ كما قال تعالى: ﴿يَا أَيُّهَا الَّذِينَ آمَنُوا كُتِبَ عَلَيْكُمُ الصِّيَامُ كَمَا كُتِبَ عَلَى الَّذِينَ مِنْ قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ﴾ وقال تعالى: ﴿وَأَنْ تَصُومُوا خَيْرٌ لَكُمْ إِنْ كُنْتُمْ تَعْلَمُونَ﴾ . فأمر ﷺ بمقاومة الشهوة واتقاء خطرها بأمرين مرتبين: الأمر الأول: الزواج عند المقدرة عليه. والثاني: الصيام لمن لم يقدر على الزواج؛ مما يدل على أنه لا يجوز للإنسان أن يترك نفسه في مدارج الخطر. وهذا كقوله تعالى: ﴿وَأَنْكِحُوا الْأَيَامَى مِنْكُمْ وَالصَّالِحِينَ مِنْ عِبَادِكُم﴾ إلى قوله تعالى: ﴿وَلْيَسْتَعْفِفِ الَّذِينَ لا يَجِدُونَ نِكَاحاً حَتَّى يُغْنِيَهُمُ اللَّهُ مِنْ فَضْلِهِ﴾ .