Exodus 3:1

3:1 Now Moses was shepherding the flock of his father-in-law Jethro, the priest of Midian, and he led the flock to the far side of the desert and came to the mountain of God, to Horeb.

Exodus 3:12

3:12 He replied, “Surely I will be with you, and this will be the sign to you that I have sent you: When you bring the people out of Egypt, you and they will serve God on this mountain.”