Musnad Ahmad 33:350

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Abu al-Zubayr, from Jabir ibn Abdullah, 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)."