Musannaf Ibn Abi Shayba 3:5099

Waki' narrated to us, he said: Abu Mawdud narrated to us, from Abd al-Rahman ibn Abi Hadrad al-Aslami, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you spits in my mosque—or he said: in the mosque—then let him dig and bury it, or let him spit into a garment until he takes it out."