Musnad Ahmad 32:1945

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj narrated to me, Shu'bah said: I heard Qatadah narrating from Anas ibn Malik, who 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 he should not spit"—Hajjaj said: "He should not expectorate"—"in front of him, nor to his right, but to his left, under his foot."