Musnad Ahmad 27:982

Bahz ibn Asad Abu al-Aswad narrated to us, Shu'bah narrated to us, Abdullah ibn Dinar narrated to us, I heard Abdullah ibn Umar saying: 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 let him cut them below the ankles."