Exodus 11:5-7

and all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts. And there shall be a great cry throughout all the land of Egypt, such as there was none like it, nor shall be like it any more. But against any of the children of Israel shall not a dog move his tongue, against man or beast: that ye may know how that the LORD doth put a difference between the Egyptians and Israel.