Economists are good at making assumptions, and one of the assumptions I've always made is that John Maynard Keynes was the source for what is usually quoted as "When the facts change, I change my mind. What do you do, sir?"
It certainly sounded like something he would have said, and the ad hominem counter-punch at the end, in particular, seemed pretty Keynesian to me.
Turns out though that I'm very probably wrong. Thus far, there's no evidence he said it. The best article I've found on it is on the Quote Investigator website ('Dedicated to Tracing Quotations'). By coincidence, the latest quote investigated on the site (as of today) is also about economics, "Teach a Parrot to Say ‘Supply and Demand’ and You Have an Economist".
This article "Keynes: He Didn’t Say Half of What He Said. Or Did He?" from the Wall Street Journal also doubts that Keynes ever said it, and to boot argues that he didn't say "The market can remain irrational longer than you can remain solvent", either (and has put up a modest prize, US$10 per quote, to anyone who can prove he did).