Musannaf Ibn Abi Shaybah 26483 — Wearing trousers

It is narrated from Abu Uthman that he said: Sayyiduna Umar (may Allah be pleased with him) wrote to Sayyiduna Abu Musa (may Allah be pleased with him): "Cut off the stirrups of the horses and leap onto them; wear sandals and take off leather socks; wear lower garments (izar) in place of trousers; shoot arrows at the targets; wear rough, coarse clothes in the Ma'addi style; and beware of the way of the non-Arabs, for the way of the non-Arabs is the worst of ways."