“Your mother was Muggle-born, of course.  Couldn’t believe it when I found out.  Thought she must have been pure-blood, she was so good.”

“One of my best friends is Muggle-born,” said Harry, “and she’s the best in our year.”

“Funny how that sometimes happens, isn’t it?” said Slughorn.

“Not really,” said Harry coldly.

Slughorn looked down at him in surprise.  “You mustn’t think I’m prejudiced!” he said.

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