Musnad Ahmad 2:11

Hassan ibn Musa narrated to us, he said: Zuhayr narrated to us, he said: Asim al-Ahwal narrated to us, from Abu Uthman, who said: The letter of Umar, may Allah be pleased with him, came to us while we were in Azerbaijan: "O Utbah ibn Farqad, beware of luxury, the attire of the people of polytheism, and the wearing of silk. For the Messenger of Allah, peace and blessings of Allah be upon him, forbade us from wearing silk." And he said: "Except like this," and the Messenger of Allah, peace and blessings of Allah be upon him, raised his two fingers for us.