Musnad Ahmad 30:491

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Humayd ibn 'Abd al-Rahman, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, saw phlegm in the qiblah of the mosque, so he scraped it off with a flint stone or something else. Then he said: "When one of you stands for prayer, let him not spit in front of him, nor to his right, for there is an angel to his right. But let him spit to his left, or under his left foot."