Exodus 11:4-7

So Moses said, “This is what the LORD says: ‘About midnight I will go throughout Egypt. Every firstborn son in Egypt will die, from the firstborn son of Pharaoh, who sits on the throne, to the firstborn son of the female slave, who is at her hand mill, and all the firstborn of the cattle as well. There will be loud wailing throughout Egypt—worse than there has ever been or ever will be again. But among the Israelites not a dog will bark at any person or animal.’ Then you will know that the LORD makes a distinction between Egypt and Israel.