Musnad Abu Ya'la al-Mawsili 115:266

Zuhayr narrated to us, Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, from Anas, from the Prophet (may Allah bless him and grant him peace), who said: "If one of you spits, let him not spit in front of him, nor to his right, but let him spit to his left or under his left foot."