The Amalekites came and attacked the Israelites at Rephidim.
References for Exodus 17:8
Moses said to Joshua, "Choose some of our men. Then go out and fight against the Amalekites. Tomorrow I will stand on top of the hill. I'll stand there with the staff of God in my hands."
So Joshua fought against the Amalekites, just as Moses had ordered. Moses, Aaron and Hur went to the top of the hill.
As long as Moses held his hands up, the Israelites were winning. But every time he lowered his hands, the Amalekites began to win.
When Moses' arms got tired, Aaron and Hur got a stone and put it under him. Then he sat on it. Aaron and Hur held his hands up. Aaron was on one side, and Hur was on the other. Moses' hands remained steady until sunset.
So Joshua destroyed the Amalekite army with swords.
Then the LORD said to Moses, "That is something to be remembered. So write it on a scroll. Make sure Joshua knows you have done it. I will completely erase the memory of the Amalekites from the earth."
Then Moses built an altar. He called it The LORD Is My Banner.
He said, "I raised my hands toward the throne of the Lord. The LORD will fight against the Amalekites for all time to come."