Musnad Abu Ya'la al-Mawsili 116:7

Ahmad narrated to us, Abu Dawud narrated to us, Shu'bah narrated to us, from Qatadah, who heard Anas bin Malik that the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, when one of you is in prayer, he is only conversing with his Lord. So let him not spit in front of him, nor to his right, but to his left, under his foot."