Hebrews 6:13-17

Now when God made a promise to Abraham, since there was no one greater than himself, he swore an oath on his own integrity to keep the promise as sure as God exists! So he said, “Have no doubt, I promise to bless you over and over, and give you a son and multiply you without measure!” So Abraham waited patiently in faith and succeeded in seeing the promise fulfilled. It is very common for people to swear an oath by something greater than themselves, for the oath will confirm their statements and end all dispute. So in the same way, God wanted to end all doubt and confirm it even more forcefully to those who would inherit his promises. His purpose was unchangeable, so God added his vow to the promise.