Acts 7:30-33

And when forty years were fulfilled, an angel appeared to him in the wilderness of mount Sinai, in a flame of fire in a bush. And when Moses saw it, he wondered at the sight: and as he drew near to behold, there came a voice of the Lord, I am the God of thy fathers, the God of Abraham, and of Isaac, and of Jacob. And Moses trembled, and durst not behold. And the Lord said unto him, Loose the shoes from thy feet: for the place whereon thou standest is holy ground.