Musnad Ahmad 25:11

Hushaym narrated to us, 'Amr ibn Dinar informed us, from Jabir ibn Zayd, from Ibn 'Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) delivered a sermon and said: "If a person in the state of 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)."