Musnad Ahmad 32:2009

Bahz narrated to us, Shu'bah narrated to us, Qatadah narrated to us, from Anas, that 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 in front of him, nor to his right, but let him spit to his left, or under his foot."