Musnad Ahmad 1101:10

Ya'qub narrated to us, Ibn Akhi Ibn Shihab narrated to us, from his uncle, 'Ata' ibn Yazid al-Laythi, then al-Junda'i, informed me that 'Ubayd Allah ibn 'Adi ibn al-Khiyar informed him, That al-Miqdad ibn 'Amr al-Kindi—who was an ally of Banu Zuhrah and was among those who witnessed Badr with the Messenger of Allah, may Allah bless him and grant him peace—informed him that he said to the Messenger of Allah, may Allah bless him and grant him peace: "What do you see if I meet a man from the disbelievers and we fight, and he strikes one of my hands with a 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, may Allah bless him and grant him peace, said: "Do not kill him." He said: "O Messenger of Allah, he cut off one of my hands, then he said that after he had cut it off." The Messenger of Allah, may Allah bless him and grant him peace, 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."