The Prose Edda 1.25

Then said Gangleri: "Great in power do these Æsir seem to me; nor is it a marvel, that much authority attends you who are said to possess understanding of the gods, and know which one men should call on for what boon soever. Or are the gods yet more?" Hárr said: "Yet remains that one of the Æsir who is called Týr: he is most daring, and best in stoutness of heart, and he has much authority over victory in battle; it is good for men of valor to invoke him. It is a proverb, that he is Týr-valiant, who surpasses other men and does not waver. He is wise, so that it is also said, that he that is wisest is Týr-prudent. This is one token of his daring: when the Æsir enticed Fenris-Wolf to take upon him the fetter Gleipnir, the wolf did not believe them, that they would loose him, until they laid Týr's hand into his mouth as a pledge. But when the Æsir would not loose him, then he bit off the hand at the place now called 'the wolf's joint;' and Týr is one-handed, and is not called a reconciler of men.