Seams in the text. Moses's father-in-law is Reuel in 2:18, Jethro in 3:1, Hobab in Judges 4:11. The mountain is Horeb here, Sinai elsewhere; Acts 7:30 sets the bush "in the wilderness of Mount Sinai." Verse 4 moves from YHWH to Elohim within one sentence. On the documentary hypothesis, Sinai belongs to J and P, Horeb to E and D.