“What happened?”
“Cracked skull,” said Madam Pomfrey, bustling up and pushing him back against his pillows.  “Nothing to worry about, I mended it at once, but I’m keeping you in overnight.  You shouldn’t overexert yourself for a few hours.”

“I don’t want to stay here overnight,” said Harry angrily, sitting up and throwing back his covers.  “I want to find McLaggen and kill him.”
“I’m afraid that would come under the heading of ‘overexertion,'” said Madam Pomfrey, pushing him firmly back onto the bed and raising her wand in a threatening manner.

Harry Potter and the Half-Blood Prince – J.K. Rowling