“The preservation of peace and the guaranteeing of man's basic freedoms and rights require courage and eternal vigilance: courage to speak and act — and if necessary, to suffer and die — for truth and justice; eternal vigilance, that the least transgression of international morality shall not go undetected and unremedied.
These lessons must be learned anew by each succeeding generation, and that generation is fortunate indeed which learns from other than its own bitter experience.”

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):
: "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 https://books.google.com.br/books?id=cEcSAAAAYAAJ&pg=PA85&redir_esc=y#v=onepage&q&f=false; F.Randolph, & Co., 1809, p. 85).
Misattributed
Variant: "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 https://books.google.com.br/books?id=VfjRAAAAMAAJ&pg=PA67&lpg=PA67&dq=%E2%80%9CEternal+vigilance+is+the+price+of+liberty.%E2%80%9D+phillips+speech+anti-slavery&source=bl&ots=H2f8ckIw9o&sig=EukDrduBdK-oQSeY_Gf-VFQ6M54&hl=en&ei=SaxmTN-0H4P98AbioIi0BA&sa=X&oi=book_result&ct=result&redir_esc=y#v=onepage&q=%E2%80%9CEternal%20vigilance%20is%20the%20price%20of%20liberty.%E2%80%9D%20phillips%20speech%20anti-slavery&f=false, Volume VIII, Chapman and Hall, 1870, p. 67).

How—and How Not—to Love Mankind http://www.city-journal.org/html/11_3_urbanities-how_and_how_no.html (Summer 2001).
City Journal (1998 - 2008)

From Does Price Fixing Destroy Liberty? (1920) by George H. Earle, Jr.

1960s, Memorial Day speech (1963)

Political questionnaire response (1952)
Context: I was taught that the American's right to be a free individual, not at the mercy of the state, was hard-won and that its price was eternal vigilance, that I too would have to be vigilant. I was made to feel that it would be a disgrace to me, as an individual, if I should not value or should give up rights that were dearly bought. I am grateful for that upbringing.

2013, "Let Freedom Ring" Ceremony (August 2013)
Context: To dismiss the magnitude of this progress -- to suggest, as some sometimes do, that little has changed -- that dishonors the courage and the sacrifice of those who paid the price to march in those years. Medgar Evers, James Chaney, Andrew Goodman, Michael Schwerner, Martin Luther King Jr. -- they did not die in vain. Their victory was great. But we would dishonor those heroes as well to suggest that the work of this nation is somehow complete. The arc of the moral universe may bend towards justice, but it doesn’t bend on its own. To secure the gains this country has made requires constant vigilance, not complacency. Whether by challenging those who erect new barriers to the vote, or ensuring that the scales of justice work equally for all, and the criminal justice system is not simply a pipeline from underfunded schools to overcrowded jails, it requires vigilance. And we'll suffer the occasional setback. But we will win these fights. This country has changed too much. People of goodwill, regardless of party, are too plentiful for those with ill will to change history’s currents.

"The Tallest Tale", p. 315
Leonardo's Mountain of Clams and the Diet of Worms (1998)

Source: The Genteel Tradition in American Philosophy (1911), p. 58

Context: : These have often been attributed to Thomas Jefferson, but also to Thomas Paine, Abraham Lincoln, and many others; Lord Denning in The Road to Justice (1988) states that the phrase originated in a statement of Irish orator John Philpot Curran in 1790: "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."