Musannaf Ibn Abi Shayba 16:639

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from al-Jurayri, from Abu ‘Uthman, who said: ‘Umar wrote to Abu Musa: "Cut the stirrups, and mount horses by leaping, and cast away the khuff (leather socks), and take to the sandals, and cast away the trousers, and wear the izar (waist-wrapper), and shoot at the targets. And upon you is the Ma’addiyyah (Arabian/Bedouin) attire, and beware of the way of the non-Arabs (Ajam), for the worst of ways is the way of the non-Arabs."