"I would never have believed it of any of you. Mr. Filch says you were up in the astronomy tower. It's one o'clock in the morning. Explain yourselves."
It was the first time Hermione had ever failed to answer a teacher's question. She was staring at her slippers, as still as a statue.

Harry Potter and the Philosopher's Stone - Chapter 15 : The Forbidden Forest
See this quote in the Pensive -