Hebrews 6:12, 15, 17

So don’t allow your hearts to grow dull or lose your enthusiasm, but follow the example of those who fully received what God has promised because of their strong faith and patient endurance.

So Abraham waited patiently in faith and succeeded in seeing the promise fulfilled.

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.