Musnad Ahmad 32:1507

'Abd al-Wahhab narrated to us, from Sa'id, from Qatadah, from Anas ibn Malik, that the Prophet, peace and blessings of Allah be upon him, said: "When one of you is in prayer, let him not spit in front of him, nor to his right, for he is conversing with his Lord. But let him spit to his left, or under his foot."