Musnad Ahmad 5:485

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from Hubayrah, from Ali, who said: "The Messenger of Allah, may Allah bless him and grant him peace, forbade me from wearing a gold ring, and from wearing Al-Qassi (a type of silk-blend garment), and Al-Mayathir (silk saddlecloths or cushions)."