Musnad Ahmad 26:562

Waki’ narrated to us, al-A’mash narrated to us, from ‘Umarah ibn ‘Umayr, from ‘Abd al-Rahman ibn Yazid, from ‘Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to us: "O assembly of youth, 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."