This is an excerpt of this book
The statement with witch Voltaire is most identified--"I disapprove of what you say, but I will defend to the death your right to say it"--is a twentieth-century invention. It was made up by Beatrice Hall (pseudonym: S. G. Tallentyre) in a book puplished in 1907. Hall never said Voltaire said it, she only said it was something he might have said, but of course that did not matter. Forever after, Voltaire was cemented to the quote and it to him.
Of late it's been claimed that unbeknownest to Beatrice Hall, Voltaire actually expressed the sentiment she ascribed to him. Ashley Montagu says he heard this from Leo Rosten. Rosten reportedly said he read it in a book by Norbert Guterman. Guterman wrote that he found the quotation in a letter Voltaire wrote on February 6, 1770, to Louis Henri Leriche. But the letter to Leriche does not include the quotation or anything like it. What it does include is Voltaire's other famous quotation: "God is always on the side of the big battalions."