Exodus 17:11-15

So it came about, when Moses held his hand up, that Israel prevailed; but when he let his hand down, Amalek prevailed.

And Moses’ hands were heavy. So they took a stone and put it under him, and he sat on it; and Aaron and Hur supported his hands, one on one side and one on the other. So his hands were steady until the sun set.

And Joshua defeated Amalek and his people with the edge of the sword.

Then the Lord said to Moses, “Write this in a book as a memorial and recite it to Joshua, that I will utterly wipe out the memory of Amalek from under heaven.”

And Moses built an altar and named it The Lord is My Banner;