Sayyiduna Abu Ayyub (may Allah be pleased with him) relates: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whatever is above the knees is awrah, and whatever is below the navel is awrah."
Sa'id bin Abi Rashid al-Basri, who narrated it, is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3237