Exodus 12:12-13

12On the night that you eat the Passover meal, I will travel through the country of Egypt. I will kill every firstborn son and every firstborn male animal. I will punish all the gods of Egypt. I am the Lord.

13When you put the blood round the doors of your houses, that will be a sign to show that you live there. When I see the blood on your house, I will pass over you. No trouble will hurt you, when I attack the people of Egypt.