Musnad Ahmad 32:1300

Muhammad ibn Bakr narrated to us, Sa'id informed us, from Qatadah, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you is in his prayer, let him not spit in front of him, nor to his right, for he is conversing with his Lord. Rather, let him spit to his left, or under his foot."