Musnad Ahmad 27:1079

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Abdullah ibn Dinar, I heard Ibn 'Umar narrating from the Messenger of Allah (may Allah bless him and grant him peace) that he said regarding the one in the state of Ihram: "If he does not find two sandals, let him wear two leather socks, and let him cut them below the two ankles."