Musnad Abu Ya'la al-Mawsili 2:75

Abu Khaythama narrated to us, Jarir narrated to us, from Asim al-Ahwal, from Abu Uthman, from Umar, who said: "Beware of wearing silk, for the Messenger of Allah (peace and blessings of Allah be upon him) forbade the wearing of silk, except like this." And he raised his index and middle fingers.