All writers are vain, selfish, and lazy, and at the very bottom of their motives there lies a mystery.
I don't know if he actually said it or not. The problem with finding quotes online is that they've often been diluted and/or misattributed so many times that you're really just getting the quote people want to repeat, not anything someone actually said. In this case, it's me who wants to repeat it.
I found it while taking a break from the screenplay to see if I could find out if this quote...
Write the story, take out all the good lines, and see if it still works
...was ever actually uttered by Hemingway. I've loved this quote since high school, but I've never found a footnote to actually tell me where it came from. I have a suspicion it's simply the stuff of legend, a pithy masterpiece that Hemingway probably would have said, had he gotten the chance, but never did.
Then again, does it matter?