Exodus 3:1, 12

Now Moses was tending the flock of Jethro his father-in-law, the priest of Midian. And he led the flock to the back of the desert, and came to Horeb, the mountain of God.

So He said, “I will certainly be with you. And this shall be a sign to you that I have sent you: When you have brought the people out of Egypt, you shall serve God on this mountain.”