Exodus 11:5-7

and all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne, even unto the first-born of the maid-servant that is behind the mill; and all the first-born of cattle. And there shall be a great cry throughout all the land of Egypt, such as there hath not been, nor shall be 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 Jehovah doth make a distinction between the Egyptians and Israel.