Here is a sample template you can use to write a letter of commendation for retirement.
To,
[Name]
Dear, [Name]
We would like to congratulate you on your retirement from [Company name]. Your contributions to the department are deeply appreciated. It was a pleasure to work alongside you.
We wish you the best for your future endeavors and fruitful retirement. We hope you are rewarded for your work.
Sincerely,
[Company name]