Sahih al-Bukhari 409
صحيح البخاري#409SahihNarrated Abu Huraira and Abu Sa`id:Allah's Messenger (ﷺ) saw some expectoration on the wall of the mosque; he took gravel and scraped it off and said, "If anyone of you wanted to spit he should neither spit in front of him nor on his right but he could spit either on 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:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 8:60