I agree! Report it to HR or your supervisor. That kind of behavior should not be tolerated in the workplace, and hopefully your employer has systems in place to prevent harassment/discrimination (they are legally required to in the US, at least). It’s also a chance for education for that coworker and maybe others who are unaware of what’s happening. I’ve found that most of the time when people are rude about my stutter, it’s usually because they don’t know what’s happening. That doesn’t excuse their behavior, but context could help prevent this kind of thing from happening again.