Harry Potter and the Order of the Phoenix
"Deep down, Fudge knows Dumbledore's much cleverer than he is, a much more powerful wizard, and in the early days of his Ministry he was forever asking Dumbledore for help and advice," said Lupin. "But it seems he's become fond of power, and much more confident. He loves being Minister for Magic and he's managed to convince himself that he's the clever one and Dumbledore's simply stirring up trouble for the sake of it" (5.183)
"I did think [Professor Snape] might be a bit better this year," said Hermione in a disappointed voice. "I mean ... you know ..." she looked around carefully; there were half a dozen empty seats on either side of them and nobody was passing the table "now he's in the Order and everything."
"Poisonous toadstools don't change their spots," said Ron sagely. "Anyway, I've always thought Dumbledore was cracked to trust Snape. Where's the evidence he ever really stopped working for You-Know-Who?" (12.161-162)
"Potter, use your common sense," snapped Professor McGonagall, with an abrupt return to her usual manner. "you know where [Professor Umbridge] comes from, you must know to whom she is reporting." (12.305)