Musnad Ahmad 30:1178

Abu Amir narrated to us, Abu Mawdud narrated to us, Abd al-Rahman ibn Abi Hadrad al-Aslami narrated to me, he said: I heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever enters this mosque and spits—or clears his throat or expectorates—let him dig in it and move it away, then let him bury it. If he does not do so, then [let him put it] in his garment, then let him go out with it."