Musnad Abu Ya'la al-Mawsili 152:124

Abu Khaythama narrated to us, Muhammad ibn Khazim narrated to us, al-A'mash narrated to us, from Ibrahim, from 'Alqama, who said: I was walking with 'Abdullah in Mina, and he met 'Uthman. He stood with him, speaking to him. 'Uthman said to him, "O Abu 'Abd al-Rahman, shall we not marry you to a young girl, perhaps she will remind you of what has passed of your time?" 'Abdullah said, "As for your saying that, the Messenger of Allah (peace and blessings of Allah be upon him) indeed 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.'"