Musnad Ahmad 32:866

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj narrated to us, he said: Shu'bah narrated to me, he said: I heard Qatadah narrating from Anas ibn Malik, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When one of you is in prayer, he is conversing with his Lord, so let him not spit" — Hajjaj said: "let him not spit" — "in front of him, nor to his right, but to his left and under his foot."