Chapter 4

From the text:

It turned out a good thing that night that they had brought little Bilbo with them, after all. For somehow, he could not go to sleep for a long while; and when he did sleep, he had very nasty dreams. He dreamed that a crack in the wall at the back of the cave got bigger and bigger, and opened wider and wider, and he was very afraid but could not call out or do anything but lie and look. Then he dreamed that the floor of the cave was giving way, and he was slipping-beginning to fall down, down, goodness knows where to.

At that he woke up with a horrible start, and found that part of his dream was true. A crack had opened at the back of the cave, and was already a wide passage. He was just in time to see the last of the ponies' tails disappearing into it. Of course he gave a very loud yell, as loud a yell as a hobbit can give, which is surprising for their size.