Qatadah narrates: I heard Anas bin Malik (may Allah be pleased with him) say that the Messenger of Allah (peace and blessings be upon him) said: "Spitting in the mosque is a sin, and its expiation is to cover it up."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3588