It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) passed by a man whose thigh was exposed. The Prophet (peace and blessings be upon him) said: "Cover your thigh, for the thigh of a man is part of the 'awrah (private area)."
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2493
Hadith Gradingحکم دارالسلام:حسن لشواهده، وهذا إسناد ضعيف، أبو يحيى القتات لين الحديث، وروى عنه إسرائيل أحاديث كثيرة مناكير جدا