Musannaf Ibn Abi Shayba 28:579

Shababah ibn Sawwar narrated to us, he said: Layth ibn Sa'd narrated to us, from Ibn Shihab, from 'Ata' ibn Yazid al-Laythi, from 'Ubayd Allah ibn 'Adi ibn al-Khiyar, 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 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.' Should I kill him, O Messenger of Allah, after he has said it?" He (the Prophet), peace and blessings of Allah be upon him, said: "Do not kill him." I said: "O Messenger of Allah, he cut off my hand, then he said that after he cut it off. Should I kill him?" He said: "Do not kill him, for if you kill him, he will be in your position before you killed him, and you will be in his position before he said the word which he said."