Plutarch, Life of Fabius Maximus
10.1-2 (tr. Ian Scott-Kilvert):
His attitude was like that of Diogenes the philosopher, who when he was once told, 'The people are mocking you', retorted 'But I am not mocked', meaning that the only people who really suffer ridicule are those who allow it to influence them and are put out by it. So Fabius endured these vexations calmly and without stress, in so far as they concerned him personally, thus confirming the truth of the philosophical maxim that a truly good man can neither be insulted nor disgraced.