Musnad Abu Ya'la al-Mawsili 110:221

Zuhayr narrated to us, Yahya ibn Abi Bukayr narrated to us, Isra'il narrated to us, from Abu Yahya, from Mujahid, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) saw the thigh of a man exposed. He said: "Cover your thigh, for the thigh of a man is his 'awrah (intimate part)."