Exodus 13:15-16

15Pharaoh refused to let us go. But the Lord killed all the firstborn sons in the country of Egypt. He killed all the firstborn males, both people and animals. That is why I offer all the firstborn male animals as a sacrifice to the Lord. But for each of my firstborn sons, I give an animal to God instead.”

16It will be like a mark on your hand or something that you fix between your eyes. It will be a sign that causes you to remember what happened. You will remember that the Lord used his great power to bring us out of Egypt.’