Do not kill him, for if you kill him, he will be in the same position as yours before you kill him, and you will be in the same position as his before saying the word that he said
عَنْ المِقْدَادِ بْنَ عَمْرٍو الكِنْدِيَّ رضي الله عنه: أَنَّهُ قَالَ لِرَسُولِ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: أَرَأَيْتَ إِنْ لَقِيتُ رَجُلًا مِنَ الكُفَّارِ فَاقْتَتَلْنَا، فَضَرَبَ إِحْدَى يَدَيَّ بِالسَّيْفِ فَقَطَعَهَا، ثُمَّ لاَذَ مِنِّي بِشَجَرَةٍ، فَقَالَ: أَسْلَمْتُ لِلَّهِ، أَأَقْتُلُهُ يَا رَسُولَ اللَّهِ بَعْدَ أَنْ قَالَهَا؟ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: «لاَ تَقْتُلْهُ» فَقَالَ: يَا رَسُولَ اللَّهِ إِنَّهُ قَطَعَ إِحْدَى يَدَيَّ، ثُمَّ قَالَ ذَلِكَ بَعْدَ مَا قَطَعَهَا؟ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: «لاَ تَقْتُلْهُ، فَإِنْ قَتَلْتَهُ فَإِنَّهُ بِمَنْزِلَتِكَ قَبْلَ أَنْ تَقْتُلَهُ، وَإِنَّكَ بِمَنْزِلَتِهِ قَبْلَ أَنْ يَقُولَ كَلِمَتَهُ الَّتِي قَالَ».
Explanation
Al-Miqdād ibn al-Aswad (may Allah be pleased with him) asked the Prophet (may Allah's peace and blessings be upon him) about the situation if he meets a man from the disbelievers during a battle and they fight with swords and the disbeliever strikes one of his hands with the sword and cuts it off, then he runs away from him and seeks protection in a tree and says: La ilāha illallāh (there is no god but Allah), is it lawful for me to kill him after cutting off my hand?
The Prophet (may Allah's peace and blessings be upon him) said to him: Do not kill him. He said: O Messenger of Allah, but he cut off one of my hands; how could I not kill him despite this?! He (may Allah's peace and blessings be upon him) said: Do not kill him because his life has become inviolable.
If you kill him after declaring his Islam, his Islam has made his life inviolable just like you, and you will have the same status as his, as it will be permissible to kill you by way of legal retribution for killing him.
Source
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