Musnad Ahmad 5:677

Yazid narrated to us, Israel ibn Yunus informed us, Abu Ishaq narrated to us, from al-Harith, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "O Ali, I love for you what I love for myself, and I dislike for you what I dislike for myself. Do not recite while you are bowing, nor while you are prostrating. Do not pray while your hair is braided, for it is the portion of the devil. Do not sit on your heels between the two prostrations, do not play with pebbles, do not spread your forearms [on the ground], do not prompt the Imam, do not wear a gold ring, do not wear silk [al-Qassi], and do not ride on silk saddlecloths [al-Mayathir]."