Blaise Pascal
Lettres provinciales, letter 16 (French: "Je n'ai fait celle-ci plus longue que parce que je n'ai pas eu le loisir de la faire plus courte"), 4 December 1656
Pascal was apologising for the length of his letter to the public. Writing something short and sharp takes more effort than writing something long.
the myth (Mark Twain): Twain, Franklin and others get credit for this joke. The earliest known wording is Pascal's, in a letter in French.
Source: Wikiquote