How James saved Snape's life

Severus was very interested i where I went every month. 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 Promfrey 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...'

MOONY, WORMTAIL, PADFOOT AND PRONGS - Pg 379