Now Moses was tending the flock of Jethro his father-in-law, the priest of Midian, and he led the flock to the far side of the wilderness and came to Horeb, the mountain of God. There the angel of the Lord appeared to him in flames of fire from within a bush. Moses saw that though the bush was on fire it did not burn up. So Moses thought, "I will go over and see this strange sight — why the bush does not burn up."
When the LORD saw that he had gone over to look, God called to him from within the bush, "Moses! Moses!"
And Moses said, "Here I am."
"Do not come any closer," God said. "Take off your sandals, for the place where you are standing is holy ground." Then he said, "I am the God of your father, the God of Abraham, the God of Isaac and the God of Jacob." At this, Moses hid his face, because he was afraid to look at God. - Exodus 3:1-6 (NIV)