Is it the wording you seek, or the idea? You will find an exegesis on the
idea in Progress and Poverty, pp. 408-21 and 433-39. George, of course, was
not the first to get the idea, either, although few have expressed it with
such vigor and conviction. Turgot, the Physiocrats, and later Walras made
much of it; Hotelling cites Jules Dupuit, too. I wouldn't doubt that a busy
scholar might find it in Cicero, Epictetus, Virgil, or Aristotle. It's too
obvious a matter for intelligent people to miss.

Mason Gaffney