… is from page 299 of the 2014 collection – The Market and Other Orders  (Bruce Caldwell, ed.) – of some of F.A. Hayek’s most influential essays; specifically, it’s from Hayek’s 1967 paper “The Results of Human Action But Not of Human Design” (footnote excluded):
The whole conception, however, that law is only what a legislator has willed and that the existence of law presupposes a previous articulation of the will of a legislator is both factually false and cannot even be consistently put into practice. Law is not only much older than legislation or even an organized state: the whole authority of the legislator and of the state derives from pre-existing conceptions of justice, and no system of articulated law can be applied except with a framework of generally recognized but often unarticulated rules of justice.