Musnad Ahmad 2:162

Yahya ibn Sa'id narrated to us, al-Taymi narrated to us, from Abu 'Uthman, who said: We were with 'Utbah ibn Farqad, and 'Umar wrote to him regarding things he would narrate from the Prophet, may Allah bless him and grant him peace. Among what he wrote to him was: That the Messenger of Allah, may Allah bless him and grant him peace, said: "No one shall wear silk in this world except he who has no share of it in the Hereafter, except like this." And he gestured with his two fingers: the index and the middle. Abu 'Uthman said: I saw that it meant the buttons of the *tayalisa* (shawls) when we saw the *tayalisa*.