Musnad Ahmad 31:836

Bishr ibn Shu'ayb ibn Abi Hamzah narrated to us, my father informed me, he said: Muhammad—meaning al-Zuhri—informed me, Humayd ibn 'Abd al-Rahman informed me, that Abu Hurayrah and Abu Sa'id al-Khudri informed him that the Messenger of Allah, peace and blessings of Allah be upon him, saw phlegm on the wall of the mosque. The Messenger of Allah, peace and blessings of Allah be upon him, took a pebble and scraped it off. Then he said: "If one of you clears his throat while he is praying, let him not clear his throat in front of his face, nor to his right, but let him spit to his left or under his left foot."