Musnad Ahmad 26:485

Ibn Numayr narrated to us, al-A‘mash informed us, from ‘Umarah ibn ‘Umayr, from ‘Abd al-Rahman ibn Yazid, who said: We entered upon ‘Abdullah [ibn Mas‘ud], and with him were ‘Alqamah and al-Aswad. He narrated a hadith—I do not think he narrated it except for my sake, as I was the youngest of the people in age—he said: We were with the Messenger of Allah (peace and blessings of Allah be upon him) as young men who possessed nothing. He said: "O assembly of young men, whoever among you is able to afford the *ba’ah* (marriage expenses/capacity), let him marry, for it is more lowering to the gaze and more protective of the private parts. And whoever is not able, then upon him is fasting, for it is a shield for him."