Hebrews 6:13-17

13For when God made promise to Abraham, since he could swear by none greater, he sware by himself, 14saying, Surely blessing I will bless thee, and multiplying I will multiply thee. 15And thus, having patiently endured, he obtained the promise. 16For men swear by the greater: and in every dispute of theirs the oath is final for confirmation. 17Wherein God, being minded to show more abundantly unto the heirs of the promise the immutability of his counsel, interposed with an oath;