Grand Mufti of Saudi Arabia Rules that Whoever Insults the Prophet Must Be an Apostate, Atheist, Christian or Jew
This is a fatwa from the website of Saudi Arabia's highest-ranking cleric, the Grand Mufti Shaykh 'Abd-al-'Aziz bin 'Abdallah Al al-Shaykh. He explains that whoever insults Muhammad must be either an apostate, an atheist, a Christian, or a Jew. In essence, the Grand Mufti is saying that whoever insults the Prophet is an infidel, as many other Islamic clerics have stated (see here, here and here, for example), but chooses to express it in a more round-about fashion. The Grand Mufti does acknowledge disagreement from scholars on whether repentance can be accepted from such a person, but declines to directly address the topic of whether he should be killed.
Answer: There is no doubt that (in regards to) insulting the Prophet, whoever is known to have faith in his heart, or whoever has one iota of faith, cannot offend the noble position [i.e. of Muhammad]. For insulting the Prophet cannot be done except by a man who is a hypocrite, if he had been called to Islam. For he is a hypocrite or, Allah forbid, an apostate from Islam and an atheist, Allah forbid, after the doctrine of the communists and the like, or a Christian or a Jew. But a Muslim, if he disobeys, even if he is a guilty sinner, he cannot insult Muhammad (peace be upon him). For this is the benevolence of Allah upon his creation: "Allah did confer a great favour on the believers when He sent among them a messenger from among themselves, rehearsing unto them the Signs of Allah, sanctifying them, and instructing them in Scripture and Wisdom, while, before that, they had been in manifest error" [Qur'an 3:164]. Some scholars say that insulting the Prophet is a sin for which no repentance is accepted, because this is the right of the Prophet if he were alive, for he (peace be upon him) (could) waive his right during his life. However, after his death it is necessary to take his right, and (therefore) no repentance is accepted from he who insults him. Some (however) believe that repentance is accepted. The important thing is that insulting him is heinous and detestable, and never comes from a heart which has tasted faith. We ask Allah for good health, and for Allah's blessings on Muhammad.
Grand Mufti Shaykh 'Abd-al-'Aziz bin 'Abdallah Al al-Shaykh, mufti.af.org.sa