Musannaf Ibn Abi Shayba 3:553

Abu Bakr narrated to us, he said: Yazid bin Harun narrated to us, from Hisham, from al-Hasan, who said: He would sometimes lay both of his feet down, and sometimes he would lay the right one down and keep the left one upright. And Muhammad, when he sat, would keep the right one upright and lay the left one down.