Musnad Ahmad 2:274

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us. And Hajjaj said: Shu‘bah narrated to me, from Qatadah, who said: I heard Abu ‘Uthman al-Nahdi say: The letter of ‘Umar came to us while we were in Azerbaijan with ‘Utbah ibn Farqad, or in al-Sham: "To proceed: The Messenger of Allah (may Allah bless him and grant him peace) forbade silk, except for this much—two fingers." Abu ‘Uthman said: We did not understand it to be anything other than the borders (stripes).