Similarly, (Isaiah 40:5) "The glory of the L–rd shall appear, and all flesh will behold as one, for the mouth of the L–rd has spoken." And where did He speak it? (Devarim 32:39) "See, now, that I — I am He, and there is no god beside Me."
Where Did God First Speak
Mekhilta Tractate Pischa 12:13