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Remember what Amalek did to you on the way!
Parasha Beshalach , Shemot 17
8. Amalek came and fought with Israel in Rephidim.
9. So Moses said to Joshua, Pick men for us, and go out and fight against Amalek. Tomorrow I will stand on top of the hill with the staff of G-d in my hand
10. Joshua did as Moses had told him, to fight against Amalek; and Moses, Aaron, and Hur ascended to the top of the hill.
11. It came to pass that when Moses would raise his hand, Israel would prevail, and when he would lay down his hand, Amalek would prevail.
12. Now Moses hands were heavy; so they took a stone and placed it under him, and he sat on it. Aaron and Hur supported his hands, one from this [side], and one from that [side]; so he was with his hands in faith until sunset.
13. Joshua weakened Amalek and his people with the edge of the sword.
14. The Lord said to Moses, Inscribe this [as] a memorial in the book, and recite it into Joshua's ears, that I will surely obliterate the remembrance of Amalek from beneath the heavens
15. Then Moses built an altar, and he named it The Lord is my miracle
16. And he said, For there is a hand on the throne of the Eternal, [that there shall be] a war for the Lord against Amalek from generation to generation.