In an actually agile project or organization, the source of truth is the user of the software, because the developers periodically and whenever else necessary talk directly with the users and listen to what they have to say, and put that into writing.

In a fake agile project or org, the source of truth is a made up document written by the PO or PM and only remotely related to what the actual user says. Devs are kept away from the user by their higher ups, who seek job guarantees.