Musnad Ahmad 26:45

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Ibrahim, from Alqamah, who said: I was walking with Abdullah in Mina, and Uthman met him, so 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 you 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 can afford the *ba'ah* (marriage/means), let him marry, for it is more lowering to the gaze and more protective of the private parts. And whoever cannot afford it, then upon him is fasting, for it is a shield for him.'"