Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) said: "When the Prophet (peace and blessings be upon him) sat in the prayer, he would place his left foot between his thigh and his shin, place his left hand on his left thigh, and place his right hand on his right thigh."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13836