Hebrews 6:17-19

In the same way God, desiring even more to demonstrate to the heirs of the promise the fact that His purpose is unchangeable, confirmed it with an oath, so that by two unchangeable things in which it is impossible for God to lie, we who have taken refuge would have strong encouragement to hold firmly to the hope set before us. This hope we have as an anchor of the soul, a hope both sure and reliable and one which enters within the veil