Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1125
Shu'ba reported: I asked Qatada about spitting, in the mosque. He said: I heard Anas b. Malik say: I heard the Messenger of Allah (may peace be upon him) say: Spitting in the mosque is a sin, and its expiation is that it should be buried.