Musannaf Ibn Abi Shaybah

Hazrat Abu Uthman (may Allah have mercy on him) narrates that Hazrat Umar (may Allah be pleased with him) wrote a letter to Hazrat Abu Musa Ash'ari (may Allah be pleased with him) and said: Separate yourselves from the camels and ride horses. And take off your leather socks and wear sandals. Leave the trousers and tie the izar (waist cloth). And abandon the pleats, and adopt the dress of the tribe of Ma'ad. And protect yourselves from the customs of the non-Arabs, because the worst customs are those of the non-Arabs.