Musannaf Ibn Abi Shayba 3:5086

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Hisham narrated to us, from Qatadah, from Anas, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Spitting in the mosque is a sin, and its expiation is that you bury it."