Hebrews 6:13-18

For when God made promise to Abraham, because he could swear by no greater, he sware by himself, saying, Surely blessing I will bless thee, and multiplying I will multiply thee. And so, after he had patiently endured, he obtained the promise. For men verily swear by the greater: and an oath for confirmation is to them an end of all strife. Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath: that by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us