Exodus 12:21-30

Then Moses summoned all the elders of Israel and said to them, “Go, select an animal from the flock according to your families, and slaughter the Passover animal.  Take a cluster of hyssop, dip it in the blood  that is in the basin, and brush the lintel and the two doorposts with some of the blood in the basin. None of you may go out the door of his house until morning. When the LORD passes through to strike Egypt and sees the blood on the lintel and the two doorposts, he will pass over the door and not let the destroyer enter your houses to strike you.  “Keep this command permanently as a statute for you and your descendants. When you enter the land that the LORD will give you as he promised, you are to observe this ceremony. When your children  ask you, ‘What does this ceremony mean to you? ’ you are to reply, ‘It is the Passover sacrifice  to the LORD, for he passed over the houses of the Israelites in Egypt when he struck the Egyptians, and he spared our homes.’ ” So the people knelt low and worshiped. Then the Israelites went and did this; they did just as the LORD had commanded Moses and Aaron. Now at midnight the LORD struck every firstborn male in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the prisoner who was in the dungeon, and every firstborn of the livestock.  During the night Pharaoh got up, he along with all his officials and all the Egyptians, and there was a loud wailing  throughout Egypt because there wasn’t a house without someone dead.