Gaslighting

My partner mutters that I’m fat when we argue and when I crack it and confront him he turns around and says he would never say that or I he said fuck and I misheard.

This has happened multiple times now. I’ve never experienced gaslighting before it has me in disbelief because I always knew it was a thing but when it actually happens to you it’s like well now what. Like what do I even say if he just denies it ?