Musnad Ahmad 27:1447

Mu'ammal narrated to us, Sufyan narrated to us, from 'Abdullah ibn Dinar, who said: I heard Ibn 'Umar say: I heard the Prophet, peace and blessings of Allah be upon him, say: "If the Muhrim (a person in the state of Ihram) does not find a pair of sandals, let him wear khuffs (leather socks), and let him cut them below the ankles."