The Once and Future King Book 3, Chapter 33
Lancelot is happy that he has been given the chance to give up his sins. He is told by a dream to get on a fabulous, beautiful boat, where he is in a state of grace and is given everything he wants. Soon, Galahad joins him there, and Lancelot learns that Galahad isn't stuck-up, he's just preoccupied with higher things. Then Galahad is taken away to find the Grail. Lancelot is taken to a chapel, and through the door he sees Bors, Galahad and Percivale, and the Grail. Lancelot is not allowed to enter.