קבלה אויף בראשית 15:17

But before the five covenants were given, there was only the covenant with fire, as it is stated (Genesis 15:17), “and behold, a smoking furnace and a flaming torch, etc.” And it is written (Genesis 15:18), “On that day the Lord made a covenant with Avram, saying.” But the fire was a covenant at first. And this is [the meaning of] Bereishit - take out fire (esh) from there, and brit (covenant) remains. Meaning to say, brit esh (a covenant of fire).