Thank You For Thank You For Hard Work
Dear {Recipient},
Thank you for the extra time and effort you put into {project/assignment} over the past {time period}. It hasn't been easy to {goal that needed to be accomplished}, but thanks to your {relevant skills} and hard work, we were able to {meet the deadline/hit our target/etc.} in a timely manner.
Keep up the good work! You've been a tremendous asset to the {team/company} from the beginning. I'm looking forward to seeing what you can achieve in the future.
Regards,
{Your Name}