Exodus 12:21-27 NIV
Then Moses summoned all the elders of Israel and said to them, “Go at once and select the animals for your families and slaughter the Passover lamb. Take a bunch of hyssop, dip it into the blood in the basin and put some of the blood on the top and on both sides of the doorframe. None of you shall go out of the door of your house until morning. When the LORD goes through the land to strike down the Egyptians, he will see the blood on the top and sides of the doorframe and will pass over that doorway, and he will not permit the destroyer to enter your houses and strike you down.
“Obey these instructions as a lasting ordinance for you and your descendants. When you enter the land that the LORD will give you as he promised, observe this ceremony. And when your children ask you, ‘What does this ceremony mean to you?’ then tell them, ‘It is the Passover sacrifice to the LORD, who passed over the houses of the Israelites in Egypt and spared our homes when he struck down the Egyptians.’ ” Then the people bowed down and worshiped.
NIV: New International Version