Abu Kathir, the freed slave of Sayyiduna Muhammad bin Jahsh (may Allah be pleased with him), narrates from Muhammad bin Jahsh, who said: The Messenger of Allah (peace and blessings be upon him) passed by Ma'mar while I was with him (peace and blessings be upon him), and his thighs were uncovered. He (peace and blessings be upon him) said: "O Ma'mar! Cover your thighs, for the thigh is 'awrah (to be covered)."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16600