(a) It is narrated from Sayyiduna Ali (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not uncover your thigh, and do not look at the thigh of anyone, living or dead."
(b) In the narration of Rawh: The Messenger of Allah (peace and blessings be upon him) came to me while my thigh was uncovered, and he (peace and blessings be upon him) said: "O Ali! Cover your thigh, for it is awrah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3232