Musnad Abu Ya'la al-Mawsili 6:136

Muhammad ibn Abd Allah ibn Numayr narrated to us, my father narrated to us, Muhammad ibn Ishaq narrated to us, from Abd Allah ibn Muhammad, from Amir ibn Sa’d, from Sa’d, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you clears his throat [spits/phlegms] in the mosque, let him bury it, so that it does not touch the skin of a believer or his garment and cause him harm."