That means if you didn't write it, then say who did. If you don't know, SAY you don't know, i.e.: "Author Unknown". If it "belongs" to a movie or a video game or a musical, at least put that.
If anyone gets defriended over the next few days, this is quite likely why. I'm just so tired of it.
ETA: If everyone in the group darn well knows the passage being quoted -- or can make a good guess -- that's an obvious exception.