"And finally, I must tell you that this year, the third-floor corridor on the right-hand side is out of bounds to everyone who does not wish to die a very painful death."
Harry laughed, but he was one of the few who did.
"He's not serious?" he muttered to Percy.
"Must be," said Percy, frowning at Dumbledore. "It's odd, because he usually gives us a reason why we're not allowed to go somewhere -- the forest's full of dangerous beasts, everyone knows that. I do think he might have told us prefects, at least."

Harry Potter and the Philosopher's Stone - Chapter 7 : The Sorting Hat
See this quote in the Pensive -