From Sahih Bukhari (Hadith 408):
Narrated 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
Reference: Bukhari 408