Musnad Ahmad 26:721

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Sulayman, from Ibrahim, from ‘Alqamah, that ‘Uthman met Ibn Mas‘ud at ‘Arafat and went aside with him and spoke to him. Then ‘Uthman said to Ibn Mas‘ud: "Do you wish for a young woman whom I may marry to you?" So ‘Abdullah ibn Mas‘ud called ‘Alqamah and narrated that the Prophet, peace and blessings of Allah be upon him, said: "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, let him fast, for indeed fasting is a *wija’* (a shield or means of curbing desire) for him."