Musnad Abu Ya'la al-Mawsili 115:329

Abu Musa narrated to us, Abd al-A’la narrated to us, Sa’id narrated to us, from Qatadah, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you is in his prayer, he must not spit in front of him, nor between his hands, for he is conversing with his Lord. Rather, [he should spit] to his left or under his left foot."