Musannaf Ibn Abi Shayba 3:5076

Waki' narrated to us, he said: Sufyan narrated to us, from Mansur, from Rib'i, from Tariq ibn Abdullah al-Muharibi, who said: The Messenger of Allah (may Allah bless him and grant him peace) said to me: "When you pray, do not spit in front of you, nor to your right. But spit to your left or under your foot."