Sahih Bukhari Volume 009, Book 083, Hadith Number 005.
Narrated By Al-Miqdad bin 'Amr Al-Kindi : An ally of Bani Zuhra who took part in the battle of Badr with the Prophet, that he said, "O Allah's Apostle! If I meet an unbeliever and we have a fight, and he strikes my hand with the sword and cuts it off, and then takes refuge from me under a tree, and says, 'I have surrendered to Allah (i.e. embraced Islam),' may I kill him after he has said so?" Allah's Apostle said, "Do not kill him." Al-Miqdad said, "But O Allah's Apostle! He had chopped off one of my hands and he said that after he had cut it off. May I kill him?" The Prophet said. "Do not kill him for if you kill him, he would be in the position in which you had been before you kill him, and you would be in the position in which he was before he said the sentence." The Prophet also said to Al-Miqdad, "If a faithful believer conceals his faith (Islam) from the disbelievers, and then when he declares his Islam, you kill him, (you will be sinful). Remember that you were also concealing your faith (Islam) at Mecca before."