This letter thanks a colleague for recommending someone for a permanent position whom you actually hired.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I just wanted to drop you a line to thank you for recommending <new hire name> when I was looking for someone to hire as a <position name>. <He/She> has worked out wonderfully, and will hopefully be a valuable employee for years to come.
Thanks again for your help, and if I can ever be of assistance to you in a similar matter, don't hesitate to contact me.
encl: <List of enclosed items goes here>