Belief in a prophet after Muhammad (s)
Q. Some individuals and groups believe there is a Prophet after Muhammad, peace be upon him. Is this acceptable?
A. It is a tenet of Islam to believe that there is no Prophet with or after the Seal of Prophets — Muhammad — Peace be upon him. There is consensus among the scholars that to deny or question such a belief constitutes apostasy (kufr).
Sources: Ibn Hazm, Maratib al-Ijma` 173 Nawawi, Sharh Sahih Muslim 10:406.