Musnad Ahmad 30:2966

Waki' narrated to us, Abu Mawdud narrated to us, from 'Abd al-Rahman ibn Abi Hadrad al-Aslami, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you spits in my mosque—or the mosque—let him dig and deepen [the hole], or let him spit into his garment until he takes it out."