Musnad Ahmad 30:413

Zayd ibn al-Hubab narrated to us, Abu Mawdud informed me, Abd al-Rahman ibn Abi Hadrad narrated to me, he said: I heard Abu Hurayrah saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "If one of you spits in the mosque, let him bury it. If he does not do so, then let him spit in his garment."