Do not kill him

Al-Miqdad ibn Al-Aswad reported: I said, “O Messenger of Allah, if I meet an unbeliever and we fight together and he severs my hand with his sword, then he hides behind a tree and claims he has submitted himself to Allah, should I kill him after he said it?” The Prophet said, “Do not kill him.” I said, “O Messenger of Allah, he cut off one of my hands and only then he said it!” The Prophet said, “Do not kill him, for if you do so, he will be in a position of sanctity and you will be in a position to suffer retaliation.”

Source: Sahih Bukhari 3794, Sahih Muslim 95

Grade: Mutaffaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Imam Muslim

عَنْ الْمِقْدَادِ بْنِ الْأَسْوَدِ أَنَّهُ أَخْبَرَهُ أَنَّهُ قَالَ يَا رَسُولَ اللَّهِ أَرَأَيْتَ إِنْ لَقِيتُ رَجُلًا مِنْ الْكُفَّارِ فَقَاتَلَنِي فَضَرَبَ إِحْدَى يَدَيَّ بِالسَّيْفِ فَقَطَعَهَا ثُمَّ لَاذَ مِنِّي بِشَجَرَةٍ فَقَالَ أَسْلَمْتُ لِلَّهِ أَفَأَقْتُلُهُ يَا رَسُولَ اللَّهِ بَعْدَ أَنْ قَالَهَا قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا تَقْتُلْهُ قَالَ فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّهُ قَدْ قَطَعَ يَدِي ثُمَّ قَالَ ذَلِكَ بَعْدَ أَنْ قَطَعَهَا أَفَأَقْتُلُهُ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا تَقْتُلْهُ فَإِنْ قَتَلْتَهُ فَإِنَّهُ بِمَنْزِلَتِكَ قَبْلَ أَنْ تَقْتُلَهُ وَإِنَّكَ بِمَنْزِلَتِهِ قَبْلَ أَنْ يَقُولَ كَلِمَتَهُ الَّتِي قَالَ

3794 صحيح البخاري كِتَاب الْمَغَازِي والله لا تذرون منه درهما

95 صحيح مسلم كِتَاب الْإِيمَانِ باب تَحْرِيمِ قَتْلِ الْكَافِرِ بَعْدَ أَنْ قَالَ لَا إِلَهَ إِلَا اللَّهُ