Sayyiduna al-Miqdad bin Amr (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, what do you think: if I meet a man from the disbelievers, and we fight, and he cuts off one of my hands with the sword, then he takes refuge from me behind a tree and says: '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, 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 / 17728