From the prologue of The Lord of the Rings:

> Now it is a curious fact that this is not the story as Bilbo first told it to his companions. To them his account was that Gollum had promised to give him a present, if he won the game; but when Gollum went to fetch it from his island he found the treasure was gone: a magic ring, which had been given to him long ago on his birthday. Bilbo guessed that this was the very ring that he had found, and as he had won the game, it was already his by right. But being in a tight place, he said nothing about it, and made Gollum show him the way out, as a reward instead of a present. This account Bilbo set down in his memoirs, and he seems never to have altered it himself, not even after the Council of Elrond. Evidently it still appeared in the original Red Book, as it did in several of the copies and abstracts. But many copies contain the true account (as an alternative), derived no doubt from notes by Frodo or Samwise, both of whom learned the truth, though they seem to have been unwilling to delete anything actually written by the old hobbit himself.