Musnad Ahmad 31:40

Sufyan 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: "Let him spit to his left, or under his left foot."