Hadith: O young men, those of you who can afford to should marry; it restrains the gaze and fortifies one's chastity. Those who cannot should fast; it controls the sexual desire

عن عبد الله بن مسعود رضي الله عنه مرفوعاً: «يا معشر الشباب، من استطاع منكم الباءة فليتزوج؛ فإنه أغض للبصر، وأحصن للفرج، ومن لم يستطع فعليه بالصوم؛ فإنه له وِجَاءٌ».
[صحيح] - [متفق عليه]

‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "O young men, those of you who can afford to should marry; it restrains the gaze and fortifies one's chastity. Those who cannot should fast; it controls the sexual desire."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Chastity and restraint are mandatory, while their opposite are prohibited, as they are products of a strong desire coupled with weak faith. As young people have the stronger desires, the Prophet (may Allah's peace and blessings be upon him) addressed them, so he could guide them to the path of virtue. Those who can afford a dowry, housing, and to support a wife should marry, because it serves to protect the eyes from prohibited sights and fortifies against debauchery. For those who cannot afford it but yearn for it, the Prophet (may Allah's peace and blessings be upon him) recommended them to fast, because this contains rewards and diminishes sexual desire by means of resisting food and drink. As a result, the soul weakens and the blood veins through which Satan travels are consequently blocked. Fasting impairs lusts just as the rock spoils ejaculation when the testicles are put between it.