Deuteronomy 32:40-41 -

These verses should be read continuously: For I lift up my hand to heaven, and say, As I live forever, if I whet my glittering sword, and if my hand take hold on judgment; I will render vengeance to mine enemies, etc. Lifting up the hand to heaven was a gesture intended to express that the person taking an oath appealed to God as a witness of his oath, and who would perish for falsehood (cf. Genesis 14:22 ); and "as the Lord liveth" was a common formula in taking an oath (cf. Numbers 14:21 ; 1 Samuel 14:39 , 1 Samuel 14:45 ; Jeremiah 5:2 ). As God could swear by none greater, he swore by himself (cf. Exodus 6:8 ; Numbers 14:30 ; Isaiah 45:23 ; Jeremiah 22:5 ; Hebrews 6:17 ), that if he did come forth to avenge himself of his enemies, he would not spare, but would do thoroughly what he had come forth to do.— Glittering sword ; literally, lightning of sword (cf. Ezekiel 21:10 (15) ).