For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself, saying, “Surely I will bless you and multiply you.” And thus Abraham, having patiently waited, obtained the promise. For people swear by something greater than themselves, and in all their disputes an oath is final for confirmation. So when God desired to show more convincingly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it with an oath, so that by two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us.
English Standard Version (ESV)