Voltaire didn't say this.
This statement was made up by
Beatrice Hall, in her
1907 book
Friends of Voltaire writing under the
pseudonym S. G. Tallentyre. Not only that, Ms. Hall specifically stated that it was something Voltaire
might have said. That is, she thought he should have said it, but didn't. You can think of the statement as Ms. Hall's summary of Voltaire's
Essay on Tolerance.
in
1963,
Norbert Guterman suggested that a
February 6,
1770 letter to Louis Henry Riche had contained:
"I detest what you write, but I would give my life to make it possible for you to continue to write."
It didn't.