I've never seen it used at the end of a letter. It's definitely not common. I would find it rather pretentious. (It reminds me of something like "So says the King" at the end of a royal decree.) I would not recommend using it in a letter that you are writing.
If you are looking for a closing you might use, you should look at the threads listed under closing letter. There are discussions of both informal and formal closings, as well as closings for specific contexts.
Edit: Apparently it was a letter from a friend, and this answers my first question, which I have deleted.