Hebrews 6:13-14

6:13 Now when God made his promise to Abraham, since he could swear by no one greater, he swore by himself, 6:14 saying, “Surely I will bless you greatly and multiply your descendants abundantly.”

Hebrews 6:17

6:17 In the same way God wanted to demonstrate more clearly to the heirs of the promise that his purpose was unchangeable, and so he intervened with an oath,