Musnad Ahmad 5:372

Hajjaj narrated to us, Layth narrated to us, Yazid ibn Abi Habib narrated to me, from Abu al-Sa'bah, from a man from Hamdan called Abu Aflah, from Ibn Zurayr, that he heard Ali ibn Abi Talib saying: The Prophet of Allah, may Allah bless him and grant him peace, took silk and placed it in his right hand, and he took gold and placed it in his left hand. Then he said: "Indeed, these two are forbidden for the males of my nation."