Musnad Abu Ya'la al-Mawsili 3:71

Ubayd Allah narrated to us, Yazid Abu Khalid al-Daysari narrated to us, Ibn Jurayj narrated to us, he said: Habib ibn Abi Thabit narrated to us, from Asim ibn Damrah, from Ali, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not expose your thigh, and do not look at the thigh of the living or the dead."