If I encountered a person amongst the infidels (in the battlefield) and he attacked me and struck me and cut off one of my hands with the sword. Then he (in order to protect himself from me) took shelter of a tree and said: I become Muslim for Allah's sake. Messenger of Allah, can I kill him after he had uttered this? The Messenger of Allah ﷺ said: Do not kill him. I (the narrator) said: Messenger of Allah, he cut off my hand and uttered this after amputating it; should I then kill him? The Messenger of Allah ﷺ said: Don't kill him, for I you kill him, verily he would be in a position where you had been before killing him and verily you would be in a position where he had been before uttering (kalima). (Using translation from Muslim 95a)  

أَنَّهُ أَخْبَرَهُ أَنَّهُ قَالَ يَا رَسُولَ اللهِ «أَرَأَيْتَ إِنْ لَقِيتُ رَجُلًا مِنَ الْكُفَّارِ فَقَاتَلَنِي فَضَرَبَ إِحْدَى يَدَيَّ بِالسَّيْفِ فَقَطَعَهَا ثُمَّ لَاذَ مِنِّي بِشَجَرَةٍ» فَقَالَ «أَسْلَمْتُ لِلَّهِ أَفَأَقْتُلُهُ يَا رَسُولَ اللهِ بَعْدَ أَنْ قَالَهَا؟» قَالَ رَسُولُ اللهِ ﷺ «لَا تَقْتُلْهُ» قَالَ فَقُلْتُ يَا رَسُولَ اللهِ إِنَّهُ قَطَعَ يَدِي ثُمَّ قَالَ ذَلِكَ بَعْدَ أَنْ قَطَعَهَا أَفَأَقْتُلُهُ؟ قَالَ رَسُولُ اللهِ ﷺ «لَا تَقْتُلْهُ فَإِنْ قَتَلْتَهُ فَإِنَّهُ بِمَنْزِلَتِكَ قَبْلَ أَنْ تَقْتُلَهُ وَإِنَّكَ بِمَنْزِلَتِهِ قَبْلَ أَنْ يَقُولَ كَلِمَتَهُ الَّتِي قَالَ»