Harry Potter and the Prisoner of Azkaban - Pg 427

  "It was stupid, thinking it was him," he muttered. "I mean, I knew he was dead."  "You think the dead we loved ever truly leave us? You think we don't recall them more clearly than ever in times of great trouble? Your father is alive in you, Harry, and shows himself most plainly when you have need of him. How else could you produce that particular Patronus? Prongs rode again last night.... You know, Harry, in a way, you did see your father last night.... You found him inside yourself."