Musnad Ahmad 992:2

Hushaym narrated to us, Hafs bin Maysarah narrated to us, from al-Ala’, from Abu Kathir, the freed slave of Muhammad bin Jahsh, from Muhammad bin Jahsh, the brother-in-law of the Prophet (peace and blessings of Allah be upon him), that the Prophet (peace and blessings of Allah be upon him) passed by Ma’mar in the courtyard of the mosque, while he was sitting with his knees drawn up to his chest (muhtabiyan), exposing a part of his thigh. The Prophet (peace and blessings of Allah be upon him) said to him: "Cover your thigh, O Ma’mar, for the thigh is an 'awrah (intimate part)."