Sayyiduna al-Miqdad (may Allah be pleased with him) reported that he said: "O Messenger of Allah, what do you think: if I meet a man from the disbelievers and he fights me, and 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." I said: "O Messenger of Allah, he cut off my hand, and then said that after cutting it off - should I kill him?" He (peace and blessings be upon him) said: "Do not kill him. 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 / 17719