Harry Potter and the Half-Blood Prince
"Your defenses," said Snape, a little louder, "must therefore be as flexible and inventive as the arts you seek to undo. These pictures" – he indicated a few of them as he swept past – "give a fair representation of what happens to those who suffer, for instance, the Cruciatus Curse" -- he waved a hand toward a witch who was clearly shrieking in agony – "feel the Dementor's Kiss" – a wizard lying huddled and blank-eyed, slumped against a wall – "or provoke the aggression of the Inferius" – a bloody mass upon the ground. (9.57)
"But she could do magic!" said Harry impatiently. "She could have got food and everything for herself by magic, couldn't she?"
"Ah," said Dumbledore, "perhaps she could. But it is my belief – I am guessing again, but I am sure I am right – that when her husband abandoned her, Merope stopped using magic. I do not think that she wanted to be a witch any longer. Of course, it is also possible that her unrequited love and the attending despair sapped her of her powers; that can happen. In any case, as you are about to see, Merope refused to raise her wand even to save her own life." (13.31)
"'You can't kid me! The asylum, that's where you're from, isn't it? "Professor," yes, of course – well, I'm not going, see? That old cat's the one who should be in the asylum. I never did anything to little Amy Beson or Dennis Bishop, and you can ask them, they'll tell you!'" (13.114)