I did a little more digging, and I think it was Pascal. Here's the most informative page that I found, which discusses the origin of the quote: http://quoteinvestigator.com/2012/04/28/shorter-letter/

Mark Twain did say “I never could make a good impromptu speech without several hours to prepare it.”

I’ve also always been fond of Winston Churchill’s version: “I’m going to give a long speech today. I haven’t had time to prepare a short one.”

I'm sure they were both quite familiar with Pascal

Give them both credit, just to be safe.

(Pascal's Wager)