Musnad Abu Ya'la al-Mawsili 116:8

Ahmad narrated to us, Abu Dawud narrated to us, he said: Shu'bah informed us, from Qatadah, Anas ibn Malik narrated to us that the Messenger of Allah (may Allah bless him and grant him peace) said: "Spitting in the mosque is a sin, and its expiation is burying it."