"Hope you have -- er -- a good holiday," said Hermione, looking uncertainly after Uncle Vernon, shocked that anyone could be so unpleasant.
"Oh, I will," said Harry, and they were surprised at the grin that was spreading over his face. "They don't know we're not allowed to use magic at home. I'm going to have a lot of fun with Dudley this summer...."

Harry Potter and the Philosopher's Stone - Chapter 17 : The Man with Two Faces
See this quote in the Pensive -