Exodus 13:15-16

The king of Egypt was stubborn and refused to let us leave. But the LORD killed every firstborn male in Egypt, both human and animal. That is why I sacrifice every firstborn male animal to the LORD, and that is why I buy back each of my firstborn sons from the LORD.’ This feast is like a mark on your hand and a reminder on your forehead to help you remember that the LORD brought us out of Egypt with his great power.”