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

Voltaire (1694–1778) French writer [pseud. of Francois-Marie Arouet]

(Spurious)
    (Source)

The words are not found in any actual Voltaire source. They belong to historian Evelyn Beatrice Hall, writing as S. G. Tallentyre, in The Friends of Voltaire (1906), describing an 1759 incident where Voltaire learned that Claude-Adrien Helvétius' book On the Mind [De l’esprit] had been burned (along with Voltaire's own On Natural Law) after condemnation by the Paris Parliament and the Sorbonne. Voltaire himself wasn't impressed by Helvétius' book, but thought a book burning was unjustified. Hall wrote:

"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.

Hall later wrote that the actual words were her own and ought not to have had quotation marks.

Variations:

• I wholly disapprove of what you say -- and will defend to the death your right to say it.
• Monsieur l’Abbé, je déteste ce que vous écrivez, mais je donnerais ma vie pour que vous puissiez continuer à écrire. [Monsieur l’abbé, I detest what you write, but I would give my life to make it possible for you to continue to write.]

For more detailed information, see Quote Origin: I Disapprove of What You Say, But I Will Defend to the Death Your Right to Say It – Quote Investigator®.