Musannaf Ibn Abi Shayba 28:394

Ibn ‘Ulayyah narrated to us, from al-Jurayri, from Abu ‘Uthman, who said: ‘Umar wrote to Abu Musa al-Ash‘ari: "Mount the riders (by jumping onto the horses), and jump onto the horses with a jump. Cast off the khuffs (leather socks) and wear sandals. Cast off the trousers and wear the izar (waist-wrapper). Practice archery at the targets, and you must wear the Ma‘addiyyah (clothing of the Arabs of Ma‘add). Beware of the ways of the non-Arabs (Ajam), for the worst of ways is the way of the non-Arabs."