Musnad Ahmad 31:878

Abu Kamil narrated to us, Ibrahim narrated to us, from Ibn Shihab, from Humayd ibn Abd al-Rahman, that Abu Sa’id informed him, and Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, saw phlegm on the wall of the mosque. He took a pebble and scraped it off. Then he said: "If one of you expectorates, let him not expectorate in front of his face, nor to his right, but let him spit to his left or under his left foot."