Musnad Ahmad 32:123

Ibn Abi ‘Adi narrated to us, from Sa‘id, and Ibn Ja‘far narrated to us, from Sa‘id, from Qatadah, from Anas, that the Prophet of Allah, peace and blessings of Allah be upon him, said: "When one of you is in prayer, he is conversing with his Lord, so let none of you spit to his right." Ibn Ja‘far said: "So let him not spit in front of him, nor to his right, but to his left, or under his feet."