This comes courtesy of The Phrase Finder:

"'I disapprove of what you say, but will defend to the death your right to say it.'

"A remark attributed to Voltaire, notably by S.G. Tallentyre [a nom de plume of E. Beatrice Hall] in The Friends of Voltaire (1907). But Tallentyre gave the words as a free paraphrase of what Voltaire wrote in his Essay on Tolerance: 'Think for yourself and let others enjoy the privelege to do so, too.' So what we have is merely Tallentyre's summary of Voltaire point of view.

"Then along comes Norbert Guterman to claim that what Voltaire did write in a letter of February [6,] 1770 to a M. Le Riche was: 'Monsieur l'Abbé, I detest what you write, but I would give my life to make it possible for you to continue to write.' So, whether or not he used the precise words, at least Voltaire believed in the principle behind them.

"From Brewer's Quotations (1994) by Nigel Rees."