Musnad Ahmad 33:1136

Musa and Yahya ibn Adam narrated to us, they said: Zuhayr narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever does not find two sandals, let him wear two leather socks (khuffayn). And whoever does not find an izar (waist-wrapper), let him wear trousers (sarawil)."