Musnad Abu Ya'la al-Mawsili 42:2

Abu Khaythama narrated to us, Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from Humayd ibn 'Abd al-Rahman, from Abu Sa'id: That the Prophet, peace and blessings of Allah be upon him, saw phlegm in the qibla of the mosque, so he scraped it off with a pebble. Then he forbade a man from spitting in front of him or to his right, and he said: "He should spit to his left or under his left foot."