Is the fasting in the saying of the Messenger of Allah, peace and blessings be upon him: "O company of youth, whoever among you can afford marriage, let him marry, for it is more effective in lowering the gaze and guarding one's chastity. And whoever cannot afford it, then let him fast, for it will be a shield for him [from sexual desire]," an encouragement or an obligation?

Allah commanded chastity and abstention from zina (fornication/adultery), and He legislated marriage as a means to achieve this. He guided those unable to marry to patience and self-restraint, in fulfillment of His saying: (And let those who find not [the means for] marriage abstain [from unlawful sexual intercourse] until Allah enriches them from His bounty). The Prophet, peace and blessings be upon him, advised fasting for those unable to marry, because it is a "shield" that curbs desire and helps in guarding against sins. The command to fast here is for recommendation, not obligation.