Musnad Ahmad 30:1115

The Messenger of Allah, may Allah bless him and grant him peace, said: "When one of you stands for prayer, let him not spit in front of him, for he is in private conversation with Allah as long as he is in his place of prayer, nor to his right, for there is an angel to his right. Rather, let him spit to his left, or under his foot, and bury it."