Musnad Ahmad 26:473

Ya'la ibn 'Ubayd narrated to us, al-A'mash narrated to us, from 'Umarah, from 'Abd al-Rahman ibn Yazid, who said: 'Abdullah said: We were with the Messenger of Allah, may Allah bless him and grant him peace, 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 indeed fasting is a shield for him."