Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Prophet (peace and blessings be upon him) passed by a man whose thigh was uncovered, and he (peace and blessings be upon him) said: "Cover it, for a man's thigh is also part of his 'awrah (the part of the body that must be covered)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11073