Sæmundur fróði, The Elder Eddas of Saemund Sigfusson…
“ No longer regarding their oaths, they, therefore, called on Thor, who immediately ran to their assistance, and lifting up his mallet Mjolnir paid the workman his wages, not with the sun and moon, and not even by sending him back to Jotunheim, for with the first blow he shattered the giant's skull to pieces, and hurled him headlong into Nifelhel. But Loki had run such a race with Svadilfari that shortly after he bore a grey foal with eight legs. This is the horse Sleipnir, which excels all horses ever possessed by gods or men. ”
