During most of the plagues God sent Egypt, a "thus says the Lord" is provided Pharaoh in order to communicate God's grace and concern for him, his people, and Israel. But there isn't always such divine communication given. God's response to Pharaoh in the ten plagues follows a pattern of threes, with a capstone in the tenth plague: God speaks a "thus says the Lord" statement in plagues one and two, four and five, and seven and eight, but is quiet in plagues three, six, and nine to allow Pharaoh to give a response. Unfortunately, Pharaoh's response is to compare God' power to his own magicians' abilities in plagues three and six, and then to threaten Moses with the death penalty in plague nine.
So when we come to plague ten, we see that the communication between God and Pharaoh has finally come to an end. Exodus 11:4-6 says, "Then Moses said, 'Thus says the LORD: "About midnight I will go out into the midst of Egypt; and all the firstborn of the land of Egypt shall die, from the firstborn of Pharaoh who sits on the his throne, even to the firstborn of the female servant who is behind the handmill, and all the firstborn of the animals. Then there shall be a great cry throughout all the land of Egypt, such as was not like it before, nor shall be like it again. But against none of the children of Israel shall a dog move its tongue, against man or beast, that you may know that the LORD does make a difference between the Egyptians and Israel"'" (NKJV).
God's word to Pharaoh had always been, "Let My people go, that they may serve Me" - more literally, "Release my people, and they will serve me." Pharaoh, however, did not want freedom for himself or Israel, so he rejected God's word to him - which left God with no other choice to give Pharaoh and Israel freedom than by taking the firstborn of all Egypt.
Friends, God's word to you today is life and freedom, so don't spurn or reject it - because "thus says the Lord."