Musannaf Ibn Abi Shayba 3:550

Ibn Fudayl and Abu Usamah narrated to us, from Yahya ibn Sa’id, from al-Qasim, from ‘Abdullah ibn ‘Abdullah, from Ibn ‘Amr, who said: "Indeed, it is from the Sunnah of the prayer that one spreads the left [foot] and sets the right [foot] upright."