Musnad Ahmad 27:626

He said: I found in my father’s book: Hajjaj narrated to us, Shu’bah narrated to me, from ‘Abdullah ibn Dinar, [who said]: I heard Ibn ‘Umar narrating from the Prophet, may Allah bless him and grant him peace, that he said: "Whoever does not find two sandals, let him wear two leather socks (khuffayn), and let him slit them or cut them below the ankles."