Evelyn Beatrice Hall was paraphrasing Voltaire's mindset, not quoting him. The closest he ever came was "Think for yourselves and let others enjoy the privilege to do so too", in Essay on Tolerance

‘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.

— Evelyn Beatrice Hall, in The Friends of Voltaire, summarizing Voltaire’s defense of a censored book (1906)