In his mouth, specifically. In Underboss he states he spit on him, in his Podcast he doesn't address it but in his Wicki they state he did.
Here's an image from Underboss and the link to Wikipedia
Personally, I lean towards him doing it, it's the same thing I would have done. If you pull a gun on me you had better kill me right then because if you don't I will kill you and add insult to injury. It is the law of the street and "The Street is always watching, She's always watching."
https://www.wikiwand.com/en/Sammy_Gravano