Zur'ah bin Abd al-Rahman bin Jarhad narrates from his father that Jarhad (may Allah be pleased with him), who was one of the People of the Suffah, said: The Messenger of Allah (peace and blessings be upon him) was sitting with us and my thigh was uncovered, so he (peace and blessings be upon him) said: "Cover it. Do you not know that the thigh is awrah?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3228