Sunan Ibn Majah 761
سنن ابن ماجه#761SahihIt 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 4:27