Musnad Abu Ya'la al-Mawsili 2:74

Ibrahim ibn al-Hajjaj al-Sami narrated to us, Hammad ibn Salamah narrated to us, from Asim al-Ahwal, from Abu Uthman al-Nahdi, who said: Umar ibn al-Khattab wrote to Utbah ibn Farqad: "Peace be upon you. To proceed: Wear the rida (upper garment) and the izar (lower garment), and discard the trousers. Wear sandals and discard the khuff (leather socks). Practice archery at targets, cut the stirrups, and leap onto horses with a leap. Adhere to the clothing of the Arabs and their way of life. Beware of affectation and the attire of the non-Arabs (Ajam). For the Messenger of Allah, peace and blessings of Allah be upon him, forbade silk, except for what is like this—three fingers—or like this—four fingers."