… is from page 46 of the 2000 Liberty Fund edition of Frederic William Maitland’s 1875 dissertation at Trinity College, Cambridge, A Historical Sketch of Liberty and Equality:
Again, laws first appeared in the history of mankind as the formulation of already existing customs, and not as the expression of the will of a superior.
(For the benefit of those who are unfamiliar with legal scholarship: no name is more respected and celebrated as an historian of law as that of Maitland.)