Musannaf Ibn Abi Shayba 35:56

Ibn 'Uyaynah narrated to us, from 'Amr, who heard Jabir saying: I heard Ibn 'Abbas saying: I heard the Prophet (may Allah bless him and grant him peace) saying: "If a person in Ihram does not find an izar (waist wrapper), let him wear trousers. And if he does not find a pair of sandals, let him wear khuffs (leather socks)."