The phrase is widely attributed to Voltaire, but was in fact coined by a biographer as a representing a distillation of Voltaire's views.
It was first used in The Friends of Voltaire, written in 1906 by Evelyn Beatrice Hall.

Hall said that she had been trying to paraphrase Voltaire's words in his Essay on Tolerance: "Think for yourselves and let others enjoy the privilege to do so too."