Musnad Abu Ya'la al-Mawsili 116:6

Ahmad al-Dawraqi narrated to us, Bahz narrated to us, Shu'bah narrated to us, Qatadah narrated to us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you is in prayer, he is conversing with his Lord. So let him not spit in front of him, nor to his right, but let him spit to his left, under his left foot."