Musnad Ahmad 33:510

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, from Jabir, that the Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you spits, let him not spit to his right, nor in front of him. But let him spit to his left, or under his foot."