Musnad Abu Ya'la al-Mawsili 115:321

Abu Musa narrated to us, Ibn Abi Adi and Abd al-A’la narrated to us, from Sa’id, from Qatadah, from Anas, 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."