On Gylfaginning

"Ganglere asked: Whose is that horse Sleipner, and what is there to say about it? Har answered: You have no knowledge of Sleipner, nor do you know the circumstances attending his birth; but it must seem to you worth the telling. In the beginning, when the town of the gods was building, when the gods had established Midgard and made Valhal, there came a certain builder and offered to make them a burg, in three half years, so excellent that it should be perfectly safe against the mountain giants and frost-giants, even though they should get within Midgard. But he demanded as his reward, that he should have Freyja, and he wanted the sun and moon besides. Then the asas came together and held counsel, and the bargain was made with the builder that he should get what he demanded if he could get the burg done in one winter; but if on the first day of summer any part of the burg was unfinished, then the contract would be void. It was also agreed that no man should help him with the work. When they told him these terms, he requested that they should allow him to have the help of his horse, called Svadilfare, and at the suggestion of Loke this was granted him.


On the first day of winter he began to build the burg, but by night he hauled stone for it with his horse. But it seemed a great wonder to the asas what great rocks the horse drew, and the horse did one half more of the mighty task than the builder. The bargain was firmly established with witnesses and oaths, for the giant did not deem it safe to be among the asas without truce if Thor should come home, who now was on a journey to the east fighting trolls. Toward the end of winter the burg was far built, and it was so high and strong that it could in nowise be taken. When there were three days left before summer, the work was all completed excepting the burg gate. Then went the gods to their judgment-seats and held counsel, and asked each other who could have advised to give Freyja in marriage in Jotunheim, or to plunge the air and the heavens in darkness by taking away the sun and the moon and giving them to the giant; and all agreed that this must have been advised by him who gives the most bad counsels, namely, Loke, son of Laufey, and they threatened him with a cruel death if he could not contrive some way of preventing the builder from fulfilling his part of the bargain, and they proceeded to lay hands on Loke. He in his fright promised with an oath that he should so manage that the builder should lose his wages, let it cost him what it would. And the same evening, when the builder drove out after stone with his horse Svadilfare, a mare suddenly ran out of the woods to the horse and began to neigh at him. The steed, knowing what sort of horse this was, grew excited, burst the reins asunder and ran after the mare, but she ran from him into the woods. The builder hurried after them with all his might, and wanted to catch the steed, but these horses kept running all night, and thus the time was lost, and at dawn the work had not made the usual progress. When the builder saw that his work was not going to be completed, he resumed his giant form. When the asas thus became sure that it was really a mountain-giant that had come among them, they did not heed their oaths, but called on Thor. He came straightaway, swung his hammer, Mjolner, and paid the workman his wages,.---not with the sun and moon, but rather by preventing him from dwelling in Jotunheim; and this was easily done with the first blow of the hammer, which broke his skull into small pieces and sent him down to Niflhel. But Loke had run such a race with Svadilfare that he some time after bore a foal. It was gray, and had eight feet, and this is the best horse among gods and men."
in Prose Edda/Gylfaginning - Snorri Sturluson