Musnad Ahmad 31:879

Sakan ibn Nafi’ narrated to us, Salih narrated to us, from al-Zuhri, Humayd ibn ‘Abd al-Rahman informed me that he heard Abu Hurayrah and Abu Sa’id al-Khudri saying: The Messenger of Allah, peace and blessings of Allah be upon him, saw phlegm in the direction of the Qiblah, so he took a pebble and scraped it off with it. Then he said: "Let no one among you spit in the direction of the Qiblah, nor to his right, but let him spit to his left or under his left foot."