Hebrews 6:16-18

Common(i) 16 Men swear by one greater than themselves, and in all their disputes the oath is final for confirmation. 17 So when God desired to show more convincingly to the heirs of the promise the unchangeable character of his purpose, he interposed with an oath, 18 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 take hold of the hope set before us.