Musnad Ahmad 5:682

Abdullah narrated to us, Ubaydullah ibn Umar al-Qawariri narrated to me, Yazid Abu Khalid al-Baysari al-Qurashi narrated to me, Ibn Jurayj narrated to us, Habib ibn Abi Thabit informed me, from Asim ibn Damrah, from Ali, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said to me: "Do not expose your thigh, and do not look at the thigh of the living or the dead."