Musnad Ahmad 33:355

Muhammad ibn Bakr narrated to us, Ibn Jurayj narrated to us, from Abu al-Zubayr, from Jabir ibn Abdullah, that the Prophet (peace and blessings of Allah be upon him) said: "When one of you prays, let him not spit in front of him, nor to his right. Let him spit to his left, or under his left foot."