صحيح مسلم

It is narrated on the authority of Miqdad b. Aswad that he said. Messenger of Allah, you just see (here is a point): 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).