Musnad Ahmad 1236:1

Yahya ibn Sa'id narrated to us, from Sufyan, from Mansur, from Rib'i, from Tariq ibn Abdullah al-Muharibi, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you pray, do not spit to your right nor in front of you, but spit behind you or to your left if it is empty; otherwise, do like this," and he rubbed it under his foot. Waki' and Abd al-Razzaq did not say: "And spit behind you," and they said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me.