Exodus 3:1, 12

One day Moses was tending the flock of his father-in-law, Jethro, the priest of Midian. He led the flock far into the wilderness and came to Sinai, the mountain of God.

God answered, “I will be with you. And this is your sign that I am the one who has sent you: When you have brought the people out of Egypt, you will worship God at this very mountain.”