Thanks to LadySeishou for finding the actual quote and otter_nanowrimo on Dreamwidth for mentioning it:

Yeah, well, the inspiration of a new story is exciting. But if you wind up not finishing ninety percent of what you start, guess what happens. After a few years you’ll have written 100 beginnings, 40 middles, and only 10 endings. Which means you’ll be great at writing beginnings, only so-so at middles, and you’ll suck at endings. Which means you will almost certainly keep faltering between the middle and the end of every story, which means you’ll keep giving up and not finishing . . . Rinse, repeat.

And that’s a hole you don’t want to fall into. So finish, even if you know this story isn’t going to win you the Nobel Prize—it’s good practice to type THE END.

It’s a great writing tip from Scott Westerfield.