Musnad Ahmad 33:1143

Sulayman ibn Dawud narrated to us, Abd al-Rahman narrated to us, from Musa ibn Uqbah, from Abu al-Zubayr, from Jabir ibn Abd Allah al-Sulami, that the Prophet, peace and blessings of Allah be upon him, said: "When one of you is praying, let him not spit in front of him, nor to his right. And let him spit to his left, or under his feet."