Chapter 5: The Book On The Mosques And The Congregations
It is narrated from Abu Hurairah and Abu Sa' eed Al-Khudri that: The Messenger of Allah saw some sputum on the wall of the mosque. He picked up a stone and scraped it off, then he said, "If anyone of you needs to spit, he should not spit in fro not of him or to his right; let him spit to his right; let him spit to his left or under his left foot."
Sunan Ibn Majah 761