The Prose Edda 1.13

Then said Gangleri: "What is the way to heaven from earth?" Then Hárr answered, and laughed aloud: "Now, that is not wisely asked; has it not been told thee, that the gods made a bridge from earth, to heaven, called Bifröst? Thou must have seen it; it may be that ye call it 'rainbow.' It is of three colors, and very strong, and made with cunning and with more magic art than other works of craftsmanship. But strong as it is, yet must it be broken, when the sons of Múspell shall go forth harrying and ride it, and swim their horses over great rivers; thus they shall proceed." Then said Gangleri: "To my thinking the gods did not build the bridge honestly, seeing that it could be broken, and they able to make it as they would." Then Hárr replied: "The gods are not deserving of reproof because of this work of skill: a good bridge is Bifröst, but nothing in this world is of such nature that it may be relied on when the sons of Múspell go a-harrying."