Gandalf gives the warning “You shall not pass!” in J.R.R. Tolkien's The Lord of the Rings.
The line occurs during the Fellowship's escape from Moria. On the Bridge of Khazad-dûm, Gandalf confronts a Balrog, a powerful demonic being from the ancient world. Gandalf blocks the bridge and commands the creature not to cross, then breaks the bridge beneath it. The Balrog's whip catches Gandalf, and both fall into the abyss.
In the novel, the confrontation continues beyond the fall. Gandalf battles the Balrog for a long period, ultimately defeating it on the peak of Zirakzigil, but he dies from the struggle. He later returns as Gandalf the White, replacing Gandalf the Grey as a transformed figure.
The quotation became even more famous through later adaptations and performances, but it originates in Tolkien's text. The scene is not at Minas Tirith or Mordor, and the opponent is a Balrog rather than Sauron. Those locations and characters belong to different parts of the story.