Musnad Abu Ya'la al-Mawsili 115:9

Khalaf and ‘Abd al-Wahid narrated to us, they said: Abu ‘Awanah narrated to us, from Qatadah, from Anas, that the Prophet (may Allah bless him and grant him peace) said: "Spitting in the mosque is a sin, and its expiation is burying it."