Rashi on Deuteronomy 32:40/רש"י על דברים ל״ב:מ׳

1כי אשא אל שמים ידי FOR I SHALL LIFT UP MY HAND TO HEAVEN — This means; for in My wrath I shall lift up My hand to Myself in an oath.

2ואמרתי חי אנכי — This is the expression of an oath: I SWEAR AS TRUE AS I LIVE [FOR EVER].