What are the hadiths indicating the obligation to kill one who insults the Prophet, peace and blessings be upon him, even if he shows repentance? And is there a text stating that his repentance is not accepted, or is it a rational deduction that his killing is a right specific to him that cannot be waived on his behalf?

The views of scholars regarding the killing of one who reviles the Prophet, peace be upon him, and then repents, can be summarized as follows: many of them hold that he should be killed even if he declares his repentance. This is the well-known opinion of Imams Malik and Ahmad, and Al-Layth ibn Sa'd, and the view of the majority of the Salaf.

Those who hold this view base their argument on the premise that reviling involves two crimes: 1. Disbelief and apostasy: The punishment for this is lifted by repentance. 2. Harming the Prophet, peace be upon him: They argue that killing is the prescribed punishment for this crime, and that it is not lifted except by the forgiveness of the one whose right was violated, namely the Prophet, peace be upon him. And this right is not forgiven by the Ummah after his death.

Among the proofs they cite are: 1. The Hadith of Ka'b ibn al-Ashraf: "Who will deal with Ka'b ibn al-Ashraf? For he has harmed Allah and His Messenger," whereupon he was given permission to be killed. 2. The story of the Jewish woman who used to insult the Prophet, peace be upon him, and a man strangled her, and the Messenger of Allah declared her blood to be of no consequence. 3. The Hadith of the two female singers who used to sing satirical verses against the Prophet, peace be upon him. The Prophet ordered them to be killed on the day of the conquest of Mecca, and they were not granted the same immunity given to others, even though a female disbelieving combatant is not killed unless she fights. This confirms that the reason for killing is the harm caused, and not merely disbelief.