Musnad Ahmad 32:1239

Ad-Dahhak ibn Makhlad narrated to us, Sa'id ibn Abi 'Aruba narrated to us, from Qatadah, that Anas ibn Malik narrated to them, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Spitting in the mosque is a sin, and its expiation is burying it."