Musnad Abu Ya'la al-Mawsili 115:127

Abu Khaythama narrated to us, Yahya narrated to us, from Shu'bah, Qatada narrated to us, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "When one of you is in his prayer, he is conversing with his Lord, so let him not spit in front of him, nor to his right, but to his left, under his foot."