"Cherish those who seek the truth but beware of those who find it" - Voltaire

Most likely Gide ("Croyez ceux qui cherchent la vérité, doutez de ceux qui la trouvent", "Believe those who seek Truth, doubt those who find it") and not Voltaire ;)

Voltaire was generally more subtle: "un bon mot ne prouve rien", a witty saying proves nothing, as he'd say.