Exodus 12:27 (KJV)

And it shall come to pass, when ye be come to the land which the Lord will give you, according as he hath promised, that ye shall keep this service.

And it shall come to pass, when your children shall say unto you, What mean ye by this service?

That ye shall say, It is the sacrifice of the Lord’s passover, who passed over the houses of the children of Israel in Egypt, when he smote the Egyptians, and delivered our houses. And the people bowed the head and worshipped.

And the children of Israel went away, and did as the Lord had commanded Moses and Aaron, so did they.

And it came to pass, that at midnight the Lord smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne unto the firstborn of the captive that was in the dungeon; and all the firstborn of cattle.