It is narrated on the authority of Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (may peace be upon him) observed:

By Him in Whose hand is the life of Muhammad, he who amongst the community of Jews or Christians hears about me, but does not affirm his belief in that with which I have been sent and dies in this state (of disbelief), he shall be but one of the inhabitants of Hell−Fire.

Sahih Muslim, Book of Faith (Book 1), Chapter 71: THE NECESSITY OF AFFIRMING THE PROPHETHOOD OF OUR APOSTLE MUHAMMAD (MAY PEACE BE UPON HIM) WHO IS THE APOSTLE SENT TO THE WHOLE OF HUMANITY, AND THE ABROGATION OF OTHER RELIGIONS WITH HIS RELIGION, Hadith No. 284