Musnad Ahmad 1101:24

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, Ibn Shihab informed me, from 'Ata' ibn Yazid al-Laythi, from 'Ubayd Allah ibn 'Adi ibn al-Khiyar, that he said: He informed me that al-Miqdad informed him that he said: "O Messenger of Allah, what is your opinion if I meet a man from the disbelievers and he fights me, and we exchange two blows, and he strikes one of my hands with the sword and cuts it off, then he takes refuge from me behind a tree and says: 'I have submitted to Allah.' Shall I kill him, O Messenger of Allah, after he has said it?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not kill him." I said: "O Messenger of Allah, he cut off one of my hands, then he said that after he had cut it off. Shall I kill him?" The Messenger of Allah said: "Do not kill him, for if you kill him, he is in your position before you killed him, and you are in his position before he said the word that he said."