As a piece of everyday speech, you know what has no single famous source, which is part of why it travels so well online. The phrase is a natural conversational opener, and short audio versions of it have circulated as reaction clips and meme sounds because they fit almost any context where someone is about to pivot, confess or joke. Creators reuse it as a familiar cue that signals a change of direction, and its brevity makes it easy to loop, caption and drop into edits without explaining the joke.