Hughes’s shooting script for the film, dated July 24, 1985 and stamped “Copyright 1985 Paramount Pictures Corporation,” is scanned at the Internet Archive ↗. The phrase does not occur anywhere in it. In that draft the bathroom monologue reaches “I do actually have a test. That wasn’t bullshit… It’s on European socialism” with no “life moves pretty fast” preamble — the scene opens instead on “I don’t care if you’re fifty five or seven, everybody needs a day off now and then.”
The draft also ends differently. Where the finished film has Ferris repeat the line from his bed, the script’s closing speech is “Yeah, life is a carousel. A great big crazy ball of pure living, breathing joy and delight” — capped, after he rolls onto his back, by his last line: “You gotta get one.” The same July 24, 1985 draft is reproduced as searchable text at IMSDb ↗, and it matches the scan. So the most quoted sentence Hughes ever wrote arrived somewhere between that draft and the print that shipped. We can show it is absent from the July 1985 script and present in the film; the documents we fetched do not say when or how it was added, and we are not going to guess.