Exodus 18:2
After Moses had sent away Zipporah, his wife, Jethro, his father-in-law, received her
Meeting with Jethro Jethro, the priest of Midian, father-in-law of Moses, heard all that God had done for Moses and for Israel, his people, when the Lord brought Israel out of Egypt.
for he said: “Take up the banner of the Lord! The Lord is at war with the Amalekites from generation to generation.”
The Lord then instructed Moses: “Write this in a book as something to be remembered, and make it known in the hearing of Joshua that I will wipe out the remembrance of Amalek from under heaven.”
As Moses’ arms grew weary, they placed a stone for him to sit on while Aaron and Hur, on either side, held up his arms, which remained steadily raised until sunset.
It happened that when Moses raised his hands, the Israelites would win, but the Amalekites would have the advantage when he lowered them.
Joshua fought the Amalekites as Moses had directed, while Moses, Aaron, and Hur went to the top of the hill.