The Quran, the Sunnah and the consensus of scholars maintain that defaming the Prophet discrediting him or mocking any act of his Sunnah constitutes major disbelief that renders a person a disbeliever. Allaah The Almighty Says (what means): {And if you ask them, they will surely say, "We were only conversing and playing." Say, "Is it Allaah and His verses and His Messenger that you were mocking?" Make no excuse; you have disbelieved after your belief.} [Quran 9:65-66]

Shaykhul-Islam, Ibn Taymiyyah mentions that this verse stands as clear evidence that mocking Allaah The Almighty, His verses or His Messenger is disbelief. Thus, insulting them is even more so. Moreover, the verse stresses that whoever seriously or jestingly discredits the Prophet becomes a disbeliever.

Ibn ‘Abbaas narrated that there was a blind man whose female servant used to speak ill of  the Prophet . Whenever the man advised or forbade her, she would not listen. One night she kept on  speaking ill of the Prophet so the man killed her. In the morning, he related what had happened  to the Prophet who had assembled the people and called for the killer to come forward. The blind man stood up and admitted that he had killed her because she used to disgrace the Prophet . The Prophet stated that there was no retaliation or blood money for someone who was killed for harming the Prophet. [Abu Daawood & An-Nasaa'i]

In addition, there are many scholars confirming the consensus on this ruling. For example, Imaam Is'haaq Ibn Raahawayh stated that Muslim scholars unanimously agreed that anyone who insults Allaah The Almighty or His Messenger or rejects anything that Allaah The Almighty sent or killed any of the prophets of Allaah The Almighty would  be considered a disbeliever. This is so even if he acknowledged all that Allaah The Almighty revealed. Muhammad Ibn Sahnoon mentioned that scholars have unanimously agreed that whoever insults or discredits the Prophet is a disbeliever who is promised a severe punishment from Allaah The Almighty. The ruling on such a person, according to the Muslim Ummah (nation), is death. Additionally, anyone who doubts his disbelief or punishment is also a disbeliever.

Thus, anyone who is proved to disparage or discredit the Messenger of Allaah   falls under this ruling. 

Allaah Knows best.