Source for the Adage: "The first liar is always believed most."

Question

In a couple of books and articles I've come across an adage, “the first liar is always believed most”:

Now, I talked to the captain first, but I want you to know that great old saying, “The first liar gets believed”, does not apply with me. I listen to everybody the same.
—Poyer, David. The Gulf, p. 322.*

And while I agree the digital age gives you additional options for setting the record straight or refuting allegations, there’s another old adage I heard once that seems to be true as well: “the first liar is always believed most.”
—Barrett, John. Comment section, “Is It a Better Time Than Ever to Pick Fights With the Press?”, Mr. Media Training Blog.

Where did this originate?

* Quotation verified using Amazon’s search inside this book feature, search text [ first liar ].

Answer 1

The First Liar Wins: You must be referring to the FLW Rule.

FLW is in effect whenever a conversational combatant asserts a fact, evidence, or conclusion that is (1) simply stated; (2) superficially plausible; and (3) emotionally appealing.

Bob Lewis writes further on InfoWorld:

It works because once uttered — and because they are simple, plausible, and appealing — any attempt to refute them sounds argumentative rather than constructive.