As per your need, plz find enclosed the Sample Retirement Letter ( Employer to Employee). If you want you can add some more sentences.
I hope this may help you out.
Dear Mr. _______
I am writing to you at the time of your retirement to thank you personally for your valuable contribution to our department throughout your term of office. Since your appointment in _________, the department's clients and your fellow employees have repeatedly reported on your devotion to duty.
Our department is sorry to lose you.
I offer you good wishes for health and happiness in your well-earned retirement.