Exodus 12:21-30 - "The First Passover"

So Moses called all the elders together and told them, “Get the lambs for your families. Kill the lambs for the Passover. Take bunches of hyssop and dip them in the bowls filled with blood. Paint the blood on the sides and top of each doorframe. No one must leave their house until morning. At the time the LORD goes through Egypt to kill the firstborn, he will see the blood on the sides and top of each doorframe. Then he will protect that house and not let the Destroyer come into any of your houses and hurt you. You must remember this command. This law is for you and your descendants forever. You must remember to do this even when you go to the land the LORD is giving you. When your children ask you, ‘Why are we doing this ceremony?’ you will say, ‘This Passover is to honor the LORD, because when we were in Egypt, he passed over the houses of Israel. He killed the Egyptians, but he saved the people in our houses.’” Then the people bowed down and worshiped the Lord. The LORD had given this command to Moses and Aaron, so the Israelites did what the Lord commanded. At midnight the LORD killed all the firstborn sons in Egypt, from the firstborn son of Pharaoh (who ruled Egypt) to the firstborn son of the prisoner sitting in jail. Also all the firstborn animals died. That night someone died in every house in Egypt. Pharaoh, his officials, and all the people of Egypt began to cry loudly.