Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [First name of employee]
It has come to my attention that this month is your anniversary with the company, and that you have been with us for [number of years] years. I would like to personally thank you for your many years of hard work and dedication.
I understand in today's business world, it is many times tempting to seek other opportunities. It is sometimes hard to keep great employees like you, and tenure with a company is something to be very proud of.
Thank you again for all your hard work, and I look forward to working with you throughout the next year, and hopefully, many years to come.