Musnad Ahmad 2:239

Abd al-Samad narrated to us, Harb narrated to us, Yahya narrated to us, from Imran ibn Hattan—as Harb reckons—that he asked Ibn Abbas about wearing silk. He said: "Ask Aisha about it." So he asked Aisha, and she said: "Ask Ibn Umar." So he asked Ibn Umar, and he said: "Abu Hafs narrated to me that the Messenger of Allah, peace and blessings of Allah be upon him, said: 'Whoever wears silk in this world shall have no share in the Hereafter.'"