"Decency, security and liberty alike demand that government officials shall be subjected to the same rules of conduct that are commands to the citizen. In a government of laws, existence of the government will be imperiled if it fails to observe the law scrupulously. Our Government is the potent, the omnipresent teacher. For good or for ill, it teaches the whole people by its example. Crime is contagious. If the Government becomes a lawbreaker, it breeds contempt for law; it invites every man to become a law unto himself; it invites anarchy. To declare that, in the administration of the criminal law, the end justifies the means -- to declare that the Government may commit crimes in order to secure the conviction of a private criminal -- would bring terrible retribution. Against that pernicious doctrine this Court should resolutely set its face." - Justice Lewis Brandeis, Olmstead v. United States (1928)
Jane Mayer used a shorter version of the quote in The Dark Side, which is where I got it from. It's worth noting that this dissent from Justice Brandeis was in regards to the government having unlawfully wiretapped citizens.