Musnad Ahmad 2:283

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, from Qatadah, from al-Sha‘bi, from Suwayd ibn Ghaflah: That ‘Umar addressed the people at al-Jabiyah and said: The Messenger of Allah (may Allah bless him and grant him peace) forbade the wearing of silk, except for the space of two, three, or four fingers, and he gestured with his palm.