The Once and Future King Book 3, Chapter 43
Lancelot comes to Guenever that night, in Meliagrance's castle. He cuts his hand breaking through the window, and in the morning, Meliagrance sees the blood. Stricken with jealousy, he accuses her of cheating on the King. When Lancelot tries to calm him down, Meliagrance refuses, and says he will fight him as a trial for Guenever. Then he secretly locks Lancelot in a dungeon, and tells everyone Lancelot has ridden ahead to the scene of the trial.