Snape Confronts Harry About Sectumsempra
Pages 524-525
After taking Draco to the hospital, Snape returns to confront Harry. He demands to know who taught him the Sectumsempra spell. Harry lies, claiming he read it in a library book, but Snape uses Legilimency to see Harry's thoughts and realizes the truth about The Half-Blood Prince. He orders Harry to bring him all his schoolbooks for inspection.
