Musnad Abu Ya'la al-Mawsili 115:246

Zuhayr narrated to us, Yazid narrated to us, Sa'id ibn Abi 'Arubah narrated to us, from Qatadah, from Anas, from the Prophet (peace and blessings of Allah be upon him), who said: "Phlegm in the mosque is a sin, and its expiation is burying it."