(1) باب قَوْلِ اللَّهِ تَعَالَى {وَمَنْ يَقْتُلْ مُؤْمِنًا مُتَعَمِّدًا جَزَاؤُهُ فَجَهَنَّمُ}
Narrated 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 Messenger (ﷺ) said, "Do not kill him." Al- Miqdad said, "But O Allah's Messenger (ﷺ)! He had chopped off one of my hands and he said that after he had cut it off. May I kill him?" The Prophet (ﷺ) 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 (ﷺ) 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."
Reference: Sahih al-Bukhari 6865, 6866
In-book reference: Book 87, Hadith 5
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 83, Hadith 5