Harry Potter and the Prisoner of Azkaban (Harry Potter, #3)

by J.K. Rowling

‘Severus was very interested in where I went every month,’ Lupin told Harry, Ron and Hermione. ‘We were in the same year, you know, and we – er – didn’t like each other very much. He especially disliked James. Jealous, I think, of James’s talent on the Quidditch pitch … anyway, Snape had seen me crossing the grounds with Madam Pomfrey one evening as she led me towards the Whomping Willow to transform. Sirius thought it would be – er – amusing, to tell Snape all he had to do was prod the knot on the tree-trunk with a long stick, and he’d be able to get in after me. Well, of course, Snape tried it – if he’d got as far as this house, he’d have met a fully grown werewolf – but your father, who’d heard what Sirius had done, went after Snape and pulled him back, at great risk to his life … Snape glimpsed me, though, at the end of the tunnel. He was forbidden to tell anybody by Dumbledore, but from that time on he knew what I was …’