Musnad Abu Ya'la al-Mawsili 115:315

Narrated to us Ubayd Allah, narrated to us Khalid, narrated to us Sa'id, from Qatadah, from Anas, regarding spitting in the mosque, that the Messenger of Allah (may Allah bless him and grant him peace) said: "It is a sin, and its expiation is burying it."