Hebrews 6:16-18

Indeed men swear [an oath] by one greater than themselves, and with them [in all disputes] the oath serves as confirmation [of what has been said] and is an end of the dispute. In the same way God, in His desire to show to the heirs of the promise the unchangeable nature of His purpose, intervened and guaranteed it with an oath, so that by two unchangeable things [His promise and His oath] in which it is impossible for God to lie, we who have fled [to Him] for refuge would have strong encouragement and indwelling strength to hold tightly to the hope set before us.