God speaks to Moses at the burning bush
Stephen recounts the burning bush and repeats God's words about holy ground, the God of Abraham, Isaac, and Jacob, Israel's oppression, and Moses' commission.
After forty years had passed, an angel appeared to Moses in the flames of a burning bush in the desert near Mount Sinai. When Moses saw it, he marveled at the sight. As he approached to look more closely, the voice of the Lord came to him: ‘I am the God of your fathers, the God of Abraham, Isaac, and Jacob.’ Moses trembled with fear and did not dare to look. Then the Lord said to him, ‘Take off your sandals, for the place where you are standing is holy ground. I have indeed seen the oppression of My people in Egypt. I have heard their groaning and have come down to deliver them. Now come, I will send you back to Egypt.’
Exodus 3:5-10 records God telling Moses to remove his sandals, identifying Himself as the God of the patriarchs, declaring that He has seen Israel's suffering, and sending Moses; Acts 7:30-34 repeats those words in Stephen's address.
