What is the ruling on a man saying: "There will be a messenger who will come in the future"?

A Muslim must believe that Muhammad (peace be upon him) is the last of the prophets and messengers, based on the Almighty’s saying: "Muhammad is not the father of any of your men, but he is the Messenger of Allah and the Seal of the Prophets." This is also supported by numerous prophetic hadiths such as: "The prophets were sealed with me," and "I am Al-'Aqib, after whom there is no prophet," and "There is no prophet after me." Whoever claims prophethood after him is a disbeliever, and whoever believes him is also a disbeliever. One who is ignorant of this should be advised; if he persists in his belief, then he is a disbeliever, apostate from Islam.