Full Idea
If someone thinks chance made the world, he should also think that if an infinite number of the letters of the alphabet were shaken together and poured out on the ground it would be possible for them to spell out the whole 'Annals' of Ennius.
Gist of Idea
Chance is no more likely to create the world than spilling lots of letters is likely to create a famous poem
Source
M. Tullius Cicero (On the Nature of the Gods ('De natura deorum') [c.44 BCE], II.93)
Book Reference
Cicero: 'The Nature of the Gods', ed/tr. McGregor,Horace [Penguin 1972], p.161