Musnad Ahmad 482:3

Narrated to us Muhammad ibn Salamah, from Ibn Ishaq, from Yazid ibn Abi Habib, from Marthad ibn Abdullah al-Yazani, from Uqbah ibn Amir al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in the Maghrib prayer while he was wearing a *farruj* made of silk—which is the *qaba* (a type of outer garment). When he finished his prayer, he removed it with a forceful motion and said: "This is not befitting for the God-fearing."