Dear Mr. X,
We are writing this letter to warn you against the fight that has taken place with a coworker during working hours in the organization. You have been punctual and have maintained good records to date. However, your conflict with Mr. XXX has not been appreciated, and we may have to take undesired steps if any such incident reoccurs in the future.
We would be glad if you maintain a good demeanor in the office and act/work as per the company's guidelines.
Thank you.
Yours sincerely,