Exodus 13:15-16

15Pharaoh stubbornly refused to let us go, so the Lord killed all the firstborn males throughout the land of Egypt, both people and animals. That is why I now sacrifice all the firstborn males to the Lord—except that the firstborn sons are always bought back.’

16This ceremony will be like a mark branded on your hand or your forehead. It is a reminder that the power of the Lord’s mighty hand brought us out of Egypt.”