Exodus 12:24-30

You shall observe this rite as a statute for you and for your sons forever. And when you come to the land that the LORD will give you, as he has promised, you shall keep this service. And when your children say to you, ‘What do you mean by this service?’ you shall say, ‘It is the sacrifice of the LORD’s Passover, for he passed over the houses of the people of Israel in Egypt, when he struck the Egyptians but spared our houses.’” And the people bowed their heads and worshiped. Then the people of Israel went and did so; as the LORD had commanded Moses and Aaron, so they did. At midnight the LORD struck down all the firstborn in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the captive who was in the dungeon, and all the firstborn of the livestock. And Pharaoh rose up in the night, he and all his servants and all the Egyptians. And there was a great cry in Egypt, for there was not a house where someone was not dead.