Harry Potter and the Chamber of Secrets
"Now, listen here, you lot," he said, glowing at them all. "We should have won the Quidditch Cup last year. We're easily the best team. But unfortunately – owing to circumstances beyond our control —"
Harry shifted guilty in his seat. He had been unconscious in the hospital wing for the final match of the previous year, meaning that Gryffindor had been a player short and had suffered their worst defeat in three hundred years.
Wood took a moment to regain control of himself. Their last defeat was clearly still torturing him. (7.38-40)
"Good, aren't they?" said Malfoy smoothly. "But perhaps the Gryffindor team will be able to raise some gold and get new brooms, too. You could raffle off those Cleansweep Fives; I expect a museum would bid for them."
The Slytherin team howled with laughter.
"At least no one on the Gryffindor team had to buy their way in," said Hermione sharply. "They got in on pure talent."
The smug look on Malfoy's face flickered.
"No one asked your opinion, you filthy little Mudblood," he spat.
Harry knew at once that Malfoy had something really bad because there was an instant uproar at his words. Flint had to dive in front of Malfoy to stop Fred and George jumping on him. (7.77-82)
For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprang up between them. A rift began to grow between Slytherin and the others. Slytherin wished to be more selective about the students admitted to Hogwarts. He believed that magical learning should be kept within all-magic families. He disliked taking students of Muggle parentage, believing them to be untrustworthy. (9.100)