Musnad Ahmad 31:191

Yahya ibn Sa'id narrated to us, from Hisham, Qatadah informed us, from Dawud al-Sarraj, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever wears silk in this world shall not wear it in the Hereafter."