“But you must remember, my fellow-citizens, that eternal vigilance by the people is the price of liberty, and that you must pay the price if you wish to secure the blessing.”

Source: Jack Faust (1997), Chapter 2, “Revelations” (p. 30)

Speech in Boston, Massachusetts (28 January 1852), Speeches Before the Massachusetts Anti-Slavery Society (1853), p. 13. The memorable and oft-quoted phrase, "eternal vigilance is the price of liberty," was not in quotation marks in the printed edition of this speech. The Home Book of Quotations, ed. Burton Stevenson, 9th ed., p. 1106 (1964), notes that "It has been said that Mr. Phillips was quoting Thomas Jefferson, but in a letter dated 14 April, 1879, Mr. Phillips wrote: '"Eternal vigilance is the price of liberty" has been attributed to Jefferson, but no one has yet found it in his works or elsewhere.' It has also been attributed to Patrick Henry."
1850s
Context: Eternal vigilance is the price of liberty — power is ever stealing from the many to the few…. The hand entrusted with power becomes … the necessary enemy of the people. Only by continual oversight can the democrat in office be prevented from hardening into a despot: only by unintermitted Agitation can a people be kept sufficiently awake to principle not to let liberty be smothered in material prosperity.

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

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).

This has been attributed to Marshall, and he might have used the phrase, but earlier uses exist:
There is an imperialism that deserves all honor and respect — an imperialism of service in the discharge of great duties. But with too many it is the sense of domination and aggrandisement, the glorification of power. The price of peace is eternal vigilance.
Leonard H. Courtney as quoted in The Life Of Lord Courtney (I920) by G. P. Gooch
Courtney's statement however is probably derived from an earlier statement with several variants:
The price of freedom is eternal vigilance.
The price of liberty is eternal vigilance.
Eternal vigilance is the price of liberty.
These have often been attributed to Thomas Jefferson, but also Thomas Paine, Abraham Lincoln, and many others; Alfred 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."
Misattributed

Source: Eldest

2000s, Where the Right Went Wrong (2004)

Special Message to the Congress on the Threat to the Freedom of Europe (1948)
Context: The recommendations I have made represent the most urgent steps toward securing the peace and preventing war. We must be ready to take every wise and necessary step to carry out this great purpose. This will require assistance to other nations. It will require an adequate and balanced military strength. We must be prepared to pay the price for peace, or assuredly we shall pay the price of war. We in the United States remain determined to seek peace by every possible means, a just and honorable basis for the settlement of international issues.