“I would have written a shorter letter, but I did not have the time.”
Blaise Pascal (1623–1662) French mathematician, physicist, inventor, writer, and Christian philosopher
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