Get your facts first, and then you can distort them as much as you please.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]
Interview (1889) by Rudyard Kipling, Elmira, New York, From Sea to Sea, Part 2, ch. 37 “An Interview with Mark Twain” (1899)
    (Source)

Broader context:

"Personally I never care for fiction or story-books. What I like to read about are facts and statistics of any kind. If they are only facts about the raising of radishes, they interest me. Just now, for instance, before you came in" -- he pointed to an encyclopædia on the shelves -- "I was reading an article about 'Mathematics.' Perfectly pure mathematics.
"My own knowledge of mathematics stops at 'twelve times twelve,' but I enjoyed that article immensely. I didn't understand a word of it: but facts, or what a man believes to be facts, are always delightful. That mathematical fellow believed in his facts. So do I. Get your facts first, and" -- the voice dies away to an almost inaudible drone -- "then you can distort 'em as much as you please."

Variant: "Get the facts first. You can distort them later."

For more discussion of this quotation, see "Get your facts first, and then you can distort them… (Barry Popik).