Sayyiduna al-Miqdad bin al-Aswad (may Allah be pleased with him) reported that he asked the Messenger of Allah (peace and blessings be upon him): "What do you think: if a man from the disbelievers fights me, and we exchange two blows each, and he cuts off one of my hands with the sword, then he takes refuge from me behind a tree and says: 'I have 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, 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 / 17722