Sayyiduna al-Miqdad bin Amr al-Kindi (may Allah be pleased with him) - who was an ally of Banu Zuhrah and took part in the Battle of Badr with the Messenger of Allah (peace and blessings be upon him) - reported that he said: "O Messenger of Allah, if I were to encounter a man from the disbelievers and we fought, and he cut off one of my hands with the sword, then he took refuge from me behind a tree and said: 'I have submitted to Allah (become Muslim)' - should I kill him after he has said it, O Messenger of Allah?" The Messenger of Allah (peace and blessings be upon him) said: "Do not kill him." He said: "O Messenger of Allah, he cut off one of my hands, and then said that after cutting it off - should I kill him?" The Messenger of Allah (peace and blessings be upon him) said: "Do not kill him, for if you kill him, he will be in the position you were in before you killed him, and you will be in the position he was in before he said the word that he said."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17721