Musnad Ahmad 27:1784

Hajjaj narrated to us, Shu'bah narrated to me, from 'Abdullah ibn Dinar, I heard Ibn 'Umar narrating from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever does not find a pair of sandals, let him wear leather socks (khuffayn), and let him slit them or cut them below the ankles."