Musnad Ahmad 32:832

Hajjaj narrated to us, he said: I heard Shu'bah narrating, he said: I said to Qatadah: Did you hear Anas narrating from the Prophet, may Allah bless him and grant him peace, that he used to say: "Spitting in the mosque is a sin"? He said: "Yes, and its expiation is burying it."