I believe you're referring to a line of Gandalf's from *An Unexpected Journey*, the first part of Peter Jackson's *Hobbit* film trilogy, which featured quite prominently in the trailer:

>**Gandalf:** You'll have a tale or two to tell of your own when you come back.

>**Bilbo:** Can you promise that I will come back?

>**Gandalf:** No. And if you do, you will not be the same.

><sup>*The Hobbit: An Unexpected Journey* (2012)</sup>

As best I recall, this line does not appear in *The Hobbit*, nor in any of Tolkien's other works. There's a faintly similar line at the end of the book, where Gandalf reflects on Bilbo's development:

>Gandalf looked at him. "My dear Bilbo!" he said. "Something is the matter with you! You are not the hobbit that you were."

><sup>*The Hobbit* Chapter 19: "The Last Stage"</sup>