Musnad Ahmad 31:560

Hajjaj narrated to us, Laith narrated to us, Aqil narrated to me, from Ibn Shihab, from Humayd ibn Abd al-Rahman, that Abu Hurairah and Abu Sa’id al-Khudri informed him that the Messenger of Allah (may Allah bless him and grant him peace) saw phlegm on the wall of the mosque. The Messenger of Allah (may Allah bless him and grant him peace) took a pebble and scraped it off. Then he said: "If one of you clears his throat, let him not spit in front of his face, nor to his right; let him spit to his left or under his left foot."