I always thought that quote came from none other than Thomas Jefferson. In fact, I used that line in the Constitutionality of Social Cost, and attempted to footnote it to Jefferson, but could not find anywhere that he said it, so I omitted a footnote.

Indeed, eternal vigilance is the price—or inthe parlance of this article, the social cost—of liberty.

Fred Shapiro at Freakonomics has a post about the origin of that famous line. It seems the line may have come from Irish statesman John Philpot Curran

The Yale Book of Quotations quotes Curran, and adds the following note:

Usually quoted as ‘Eternal vigilance is the price of liberty,’ which has been attributed to Thomas Jefferson, but no one has ever found this in his writings. Atkinson’s Casket, Sept. 1833, has ‘The price of liberty is eternal vigilance.’

Since the publication of the YBQ, the monticello.org website has pushed “Eternal vigilance is the price of liberty” back to 1809, in Thomas U. P. Charlton‘s, The Life of Major General James Jackson (Augusta, Ga.: Randolph & Co., 1809).

So Life, Liberty, & Property/Pursuit of Happiness is from Locke. Eternal Vigilance is from Curran. Interesting.