
“I would have written a shorter letter, but I did not have the time.”
Often misattributed to Twain, this is actually by Blaise Pascal, "Lettres provinciales", letter 16, 1657:
Je n'ai fait celle-ci plus longue que parce que je n'ai pas eu le loisir de la faire plus courte.
Translation: I have only made this [letter] longer, because I have not had the opportunity to make it shorter.
Misattributed
Source: The Provincial Letters