A phrase can be used so frequently in a novel that quoting it becomes a coded signal you’re a fan of the book. The best example is ‘So it goes’ from SH5. It’s a motto, a tattoo, a stand alone Reddit post comment. For those who can’t place the quote it also serves as a cultural shibboleth.
But the only other ones I can think of are ’Es muss sein’ from TULoB and ‘Oh my valves!’ from CoD. (Why are all three words?!) But there must be others. I don’t mean famous one liners, I’m talking about multiple uses of a phrase. Can anyone mention another?