Musnad Ahmad 2:100

Yahya narrated to us, from ‘Abd al-Malik, ‘Abdullah, the freed slave of Asma’, narrated to us, saying: Asma’ sent me to Ibn ‘Umar, saying that it had reached her that you forbid three things: the emblem (or mark) on a garment, the purple saddlecloth, and fasting the whole of Rajab. He said: As for what you mentioned regarding fasting Rajab, then what about the one who fasts forever? And as for what you mentioned regarding the emblem on a garment, I heard ‘Umar, may Allah be pleased with him, say: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever wears silk in this world will not wear it in the Hereafter."