Another Story for Another Time

When someone gives an Exposition, they'll often mention something that pertains to a certain event. Questioning the character about it would elicit this response.

Due to The Law of Conservation of Detail, this story may later turn out to be relevant.

Film

 * Used as a Sequel Hook in the ending of the 1982 Conan the Barbarian, where narration by Mako tells us that in time, Conan "became a king by his own hand, and this story shall also be told."

Literature

 * Artemis Fowl: "...but that's another story" becomes something of a catchphrase for the first book's narrator. Most of these stories are eventually told as sideplots in books 2 and 3.
 * The Neverending Story ends many of its chapters with a reference to what happened to a side character who had been important in the chapter, but "that is another story and will be told another time."

Live Action TV

 * Often happens on How I Met Your Mother, usually refering to events in future episodes. Subverted in one episode when Older Ted says this of a security guard and his band, then decides that he'll probably never get to it and gives the Cliff Notes version ("They had one song, it didn't suck, end of story.")
 * Due South used to use a variant a lot, with Fraser going "...But that's not important, what is important..."

Tabletop RPG

 * Dungeons and Dragons module DA 1 Adventures in Blackmoor. The DM Background section is written like someone telling a story. At one point it starts describing the destruction of Blackmoor, but then says "But of the sinking of that fabled land - another time! Today, we speak of other things."

Western Animation
"Lisa: How come we never heard about this until today? Bart: Yeah, and what happened to the money you made? Lisa: Why haven't you hung up your gold records? Bart: Since when could you write a song? Homer: (laughs) There are perfectly good answers to those questions, but they'll have to wait for another night."
 * In "Homer's Barbershop Quartet" from The Simpsons, Bart and Lisa still have questions after Homer finishes his tale of a Beatles-like career.