Exodus 12:25–27

And when you come to the land which 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 slew the Egyptians but spared our houses.'” And the people bowed their heads and worshiped.