Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of employee]
I just wanted to let you know that your efforts have not gone unnoticed. From upper management on down, people are talking about your strong contributions to this organization.
On a personal level, I'd like to thank you for your perseverance and dedication. I know you don't have an easy job. Yet you approach it with an unwavering work ethic and an overt professionalism.
Keep up the great work!