Musnad Ahmad 32:947

Yahya ibn Sa'id narrated to us, from Hisham, Qatadah narrated to us, and Waki' narrated to us, Hisham narrated to us, from Qatadah, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "Spitting in the mosque is a sin, and its expiation is to bury it."