In Carl Boyer's 2nd Edition of "History of Mathematics", Wiley, 1991, pg. 420: "... Jean Bernoulli signed a pact under which, in return for a regular salary, he agreed to send L'Hospital his discoveries in mathematics, to be used as the marquis wished." The story is unreferenced.
P.S. I have just read Avinoam Mann's letter with the same quote. Still, what is the correct story?