Musnad Ahmad 992:3

Sulayman ibn Dawud narrated to us, Isma'il narrated to us, al-'Ala' informed me, from Abu Kathir, from Muhammad ibn Jahsh, who said: The Prophet, peace and blessings of Allah be upon him, passed by Ma'mar while I was with him, and his thighs were uncovered. He said: "O Ma'mar, cover your thighs, for the thighs are an 'awrah (intimate part)."