I performed a holdout cross-validation analysis on a multilevel model fit. The purpose of this was to show that we didn't have a problem with over-fitting, for which it worked just fine. Now we are writing it up for publication and I need a citation to support my methodology. I am looking for a good canonical statistical reference, ideally a book, that does a nice job explaining why holdout cross-validation is a real thing that people do and makes sense in this application. The paper will be published in a biological journal, so I am looking to point non-statistical types to a general reference. Somehow, none of my books seem to quite do it. The Wikipedia entry would be perfectly adequate for my purpose, but I'd rather not cite Wikipedia. Any suggestions?