It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The thigh is awrah."
These are sahih (authentic) chains that can be used as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3231