Source, in full
Often attributed to Jefferson, no original source for this has been found in his writings, and the earliest established source for similar remarks are those of John Philpot Curran in a speech upon the Right of Election (1790), published in Speeches on the late very interesting State trials (1808)
Notes from Wikiquote
"It is the common fate of the indolent to see their rights become a prey to the active. The condition upon which God hath given liberty to man is eternal vigilance; which condition if he break, servitude is at once the consequence of his crime and the punishment of his guilt." | In a biography of Major General James Jackson published in 1809, author Thomas Charlton wrote that one of the obligations of biographers of famous people is | "fastening upon the minds of the American people the belief, that the price of liberty is eternal vigilance " (in Thomas Usher Pulaski Charlton, The life of Major General James Jackson; F.Randolph, & Co., 1809, p. 85).
Variants
"Eternal vigilance is the price of liberty; power is ever stealing from the many to the few" (from a speech by Wendell Phillips at the Massachusetts Anti-Slavery Society on January 28, 1852; quoted by John Morley, ed., The Fortnightly, Volume VIII, Chapman and Hall, 1870, p. 67).