Who said?

[TorahBoy]

"I dissagree wholeheartedly with what you are saying, but would defend to the death your right to say it"

[JoolsTwo]

Voltaire?

[Stephen_G]

Jim is right.

http://www.brainyquote.com/quotes/quotes/v/voltaire109645.html

[Quizmonster]

These words are often attributed to Voltaire, though the writer, Evelyn Beatrice Hall, was not actually QUOTING him when she used them 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 was quoted in Saturday Review (May 1935) 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".
So, it seems to be just a long-standing error that has taken on the mantle of 'truth'. When something is once misquoted in one publication, it's amazing how often most other similar publications simply follow suit. No doubt it is what Voltaire THOUGHT, but not necessarily what he SAID/WROTE.