If a Muslim apostates, he will be called for three days to repent, then if he repents, [and therefore re-embraces Islam] this is acceptable from him, otherwise he should be killed, and this is the religious ruling according to Islamic Law. The Prophet said: "Whoever changes his religion, kill him." [Al-Bukhaari]  Moreover, Imaam Maalik reported that 'Umar said regarding a man who apostated and was killed: "Why did you not lock him for three days, and feed him a loaf of bread every day and call him to repent, as he might have repented and returned to Allaah's Commandments."

Therefore, whoever apostates, must be called to repent for three days, and then if he repents, that is acceptable from him, otherwise he should be killed. However, it should be noted that it is only the ruler who may inflict this corporal punishment determined by Islamic Law and other corporal punishments, and it is not permissible for individuals to carry out this punishment without the permission of the ruler as this leads to anarchy and other harms. If there is no ruler who may inflict corporal punishments determined by Islamic Law, it is an obligation on the Muslims, after making sure that the conditions of ruling that one has apostated are fulfilled and that there are no other impediments, to treat such person as a non-Muslim, they should not marry him, they should not inherit him, and his wife should divorce him, and other rulings that apply to him. However, one has to be keen on calling him to Allaah as he might repent and return to Him. For more benefit, please refer to Fatwa: 85488.

Allaah Knows best.