When did Harry find out about Sirius almost killing Snape?

Question

I'm trying to find, when Harry found out that Sirius nearly killed Snape by making him go down the Whomping Willow to where Remus was in werewolf form?

Answer 1

It's at the very end of Chapter 18, Moony, Wormtail, Padfoot, and Prongs, of the third book, HP and the Prisoner of Azkaban:

"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 field... anyway Snape had seen me crossing the grounds with Madam Pomfrey one evening as she led me toward 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 by Dumbledore to tell anybody, but from that time on he knew what I was..."

"So that's why Snape doesn't like you," said Harry slowly, "because he thought you were in on the joke?"

"That's right," sneered a cold voice from the wall behind Lupin.

Severus Snape was pulling off the Invisibility Cloak, his wand pointing directly at Lupin.

_{(@Slytherincess This time I did copy-paste, from here, rather than transcribing by hand.)}