Under Shari’ah law, the penalty for a Muslim who departs from Islam is death.
A book published in the United States, Reliance of the Traveller: A Classic Manual of Islamic Sacred Law, is a translation of 'Umdat al-salik by Ibn Naqib (died 1368 A.D.). This Shari’ah legal manual explicitly presents the punishment for Muslim who leaves Islam. It states that, if a sane person leaves Islam voluntarily, the person is to be killed by the proper Islamic authority. However, if someone kills an apostate on his own, the killer does not face the capital punishment, because he has killed someone who deserves to die. Although this book is published in the United States, it has not been denounced by Muslims. In fact, it is featured by many respected Islamic bookstores without a word warning that it advocates murderous acts contrary to human rights and constitutional law. Many fundamentalist Muslims believe that, even within Western nations, Shari’a law is supreme over all human legal systems.
o8.1 "When a person who has reached puberty and is sane voluntarily apostatizes from Islam, he deserves to he killed."
o8.2 "In such a case, it is obligatory for the caliph (A: or his representative) to ask him to repent and return to Islam. If he does, it is accepted from him, but if he refuses, he is immediately killed."
o8.4 "There is no indemnity for killing an apostate (O: or any expiation, since it is killing someone who deserves to die)."
o8.5 "If he apostatizes from Islam and returns several times, it (O: i.e. his return to Islam, which occurs when he states the two Testifications of Faith (def: o8.7(12))) is accepted from him, though he is disciplined (o17)."
(Quotations from: Ahmad ibn Naqib al-Misri, Reliance of the Traveller: A Classic Manual of Islamic Sacred Law, Translated by Nuh Ha Mim Keller, Amana Publications, Beltsville, MD, 1997 Revised Edition, p. 595-598.)
Sura 4:88-89 "Then what is the matter with you that you are divided into two parties about the hypocrites? Allah has cast them back (to disbelief) because of what they have earned. Do you want to guide him whom Allah has made go astray? And he whom Allah has made to go astray, you will never find for him any way (of guidance) 89 They wish that you reject (Faith), and thus that you all become equal (like one another). So, take not Auliya (protectors or friends) from them, till they emigrate in the way of Allah (to Muhammad). But if they turn back (from Islam), take (hold of) them and kill [q-t-l] them wherever you find them."
(Note: the Arabic verb 'qatala' is used (root is q-t-l), and this word means exclusively to fight, kill, war, battle, or slaughter.)
Bukhari records this tradition traced back to Muhammad himself in a legal context. It gives three reasons for shedding a Muslim’s blood. One of them is apostasy:
"Allah's Apostle said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Apostle, cannot be shed except in three cases: In Qisas [like-for-like punishment] for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims." (Bukari)
As another example from Bukhari’s collection, this hadith promises a reward on the Day of Resurrection for killing apostates in the last days:
"No doubt I heard Allah's Apostle saying, "During the last days there will appear some young foolish people who will say the best words but their faith will not go beyond their throats (i.e. they will have no faith) and will go out from (leave) their religion as an arrow goes out of the game. So, wherever you find them, kill them, for whoever kills them shall have reward on the Day of Resurrection." (Bukhari)
Maududi cites two hadiths that he considers reliable. Both say that a Christian or Christians converted to Islam, disliked it, and then switched back to Christianity. Ali is Muhammad’s cousin and son-in-law and fourth Caliph (ruled AD 656-660), who was assassinated while praying in a mosque. How does he treat these Christians? Tolerantly?:
"A man who was formerly a Christian, then was Muslim, and again became a Christian was brought before Ali. Ali asked him: What is the cause of your conduct? He replied: I have found the religion of the Christians better than your religion. Ali asked: What is your belief about Jesus? He said: He is my Lord (Rabb); or else he said: He is Lord of Ali. Hearing this, Ali ordered that he be executed."
These Christians were martyred after they realized that their first religion, Christianity, was far more excellent than all other religions, even Islam. Note what Ali does to their children:
"Ali was informed about a group of Christians who had become Muslims and then became Christians again. Ali arrested them, summoned them before himself and enquired about the truth of the matter. They said: We were Christians. Then we were offered the choice of remaining Christians or becoming Muslims. We chose Islam. But now it is our opinion that no religion is more excellent than our first religion. Therefore we have become Christians now. Hearing this, Ali ordered these people to be executed and their children enslaved."
In one long hadith, Malik first lays the foundation that execution is legal.
"Yahya related to me . . . that the Messenger of Allah . . . said, "If someone changes his religion—then strike off his head!"