Sayyiduna al-Miqdad bin Amr al-Kindi (may Allah be pleased with him) - who took part in the Battle of Badr - reported that he said: "O Messenger of Allah, what do you think: if I meet a man from the disbelievers and I fight him, and he cuts off my hand, 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?" He (peace and blessings be upon him) said: "Do not kill him, for if you kill him, you will be in the position he was in before he said the word that he said."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17723