Hebrews 6:16-18

16Men swear by someone greater than themselves, and their oath serves as a confirmation to end all argument. 17So when God wanted to make the unchanging nature of His purpose very clear to the heirs of the promise, He guaranteed it with an oath. 18Thus by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope set before us may be strongly encouraged.