Musnad Ahmad 1236:3

Ubaydah ibn Humayd narrated to us, he said: Mansur narrated to me, from Rib'i ibn Hirash, from Tariq ibn Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not spit in front of you, nor to your right, but [spit] to your left or under your foot, then rub it."