You don't need documentation to address this. You need to stand up for yourself. That's the way to deal with any bully. Address it to the coworker on Monday. It's unprofessional and it needs to stop. i.e. Stand up for yourself. Also, speak to your supervisor and inform them of your concerns. And don't tolerate the interruptions. If it continues, double down and interrupt them. "I wasn't finished speaking." Firmly. Can you elaborate on how they are being patronizing? If you're going to seek out a speech therapist, do so to work on your stutter.