Ubaydullah bin Adi bin al-Khiyar (may Allah have mercy on him) said: Sayyiduna al-Miqdad (may Allah be pleased with him) said to the Prophet (peace and blessings be upon him): "O Prophet 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 Prophet of Allah?" He (peace and blessings be upon him) said: "Do not kill him." He said: "He cut off my hand, and then said that after cutting it off?" The Prophet 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 / 17720