Exodus 3:1, 12

Now Moses was keeping the flock of Jethro (Reuel) his father-in-law, the priest of Midian; and he led his flock to the west side of the wilderness and came to Horeb (Sinai), the mountain of God.

And God said, “Certainly I will be with you, and this shall be the sign to you that it is I who have sent you: when you have brought the people out of Egypt, you shall serve and worship God at this mountain.”