“I may not agree with what you have to say but I will fight you to the death for the right to fight you to the death.”

To a British military officer (August 1780), as quoted in Washington and the Generals of the American Revolution (1856), by Rufus Wilmot Griswold, William Gilmore Simms, and Edward Duncan Ingraham. J.B. Lippincott, p. 271. Also quoted in "Death of Baron De Kalb" https://books.google.com/books?id=k2QAAAAAYAAJ&pg=PA234&lpg=PA234&dq=%22I+thank+you+sir+for+your+generous+sympathy,+but+I+die+the+death+I+always+prayed+for:+the+death+of+a+soldier+fighting+for+the+rights+of+man%22&source=bl&ots=-93hJzoCYU&sig=tAag8ObQI-ZjiII56viczov02wM&hl=en&sa=X&ei=VlYVVcuJI4KmNsazgYgL&ved=0CCUQ6AEwAQ#v=onepage&q=%22I%20thank%20you%20sir%20for%20your%20generous%20sympathy%2C%20but%20I%20die%20the%20death%20I%20always%20prayed%20for%3A%20the%20death%20of%20a%20soldier%20fighting%20for%20the%20rights%20of%20man%22&f=false (1849), by Benjamin Franklin Ells, The Western Miscellany, Volume 1, p. 233. These were reportedly his last words.
1780s

Source: The Friends of Voltaire (1906), Ch. 7 : Helvetius : The Contradiction, p. 199; because of quote marks around the original publication of these words, they are often attributed to Voltaire, though Hall was not actually quoting him but summarizing his attitude with the expression. The statement was widely popularized when misattributed to Voltaire as a "Quotable Quote" in Reader's Digest (June 1934), but in response to the misattribution, Hall had been quoted in Saturday Review (11 May 1935), p. 13, as stating: I did not mean to imply that Voltaire used these words verbatim and should be surprised if they are found in any of his works. They are rather a paraphrase of Voltaire's words in the Essay on Tolerance — "Think for yourselves and let others enjoy the privilege to do so too."
The paragraph in which the statement first appears reads:
Context: 'What a fuss about an omelette!' he had exclaimed when he heard of the burning. How abominably unjust to persecute a man for such an airy trifle as that! "I disapprove of what you say, but I will defend to the death your right to say it," was his attitude now.

Variants:
Monsieur l’abbé, I detest what you write, but I would give my life to make it possible for you to continue to write.
I wholly disapprove of what you say—and will defend to the death your right to say it.
Though these words are regularly attributed to Voltaire, they were first used by Evelyn Beatrice Hall, writing under the pseudonym of Stephen G Tallentyre in The Friends of Voltaire (1906), as a summation of Voltaire's beliefs on freedom of thought and expression. http://books.google.it/books?id=j3kGAQAAIAAJ&q=%22I+disapprove+of+what+you+say,+but+I+will+defend+to+the+death+your+right+to+say+it%22+intitle:%22The+Friends+of+Voltaire%22&dq=%22I+disapprove+of+what+you+say,+but+I+will+defend+to+the+death+your+right+to+say+it%22+intitle:%22The+Friends+of+Voltaire%22&hl=it&ei=6J3uTbDYKcLX8gOnkLGTBw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCsQ6AEwAA
Another possible source for the quote was proposed by Norbert Guterman, editor of "A Book of French Quotations," who noted a letter to M. le Riche (6 February 1770) in which Voltaire is quoted as saying: "Monsieur l'abbé, I detest what you write, but I would give my life to make it possible for you to continue to write" ("Monsieur l'abbé, je déteste ce que vous écrivez, mais je donnerai ma vie pour que vous puissiez continuer à écrire"). This remark, however, does not appear in the letter.
Misattributed

Source: Ceremony

Source: The Red Pyramid

Source: The Beautiful and Damned