Sayyiduna al-Miqdad bin Amr (may Allah be pleased with him) - who was the horseman of the Messenger of Allah (peace and blessings be upon him), a man of Kindah and an ally of Banu Zuhrah - said: I said: "O Messenger of Allah, what do you think: if I were to encounter a man from the polytheists and he and I exchanged two blows each, and he cut off one of my hands, and I struck him, then I gained the upper hand to kill him, and when I was about to lunge at him with my weapon he said: 'There is no true god except Allah' - should I kill him or leave him?" He (peace and blessings be upon him) said: "Rather, leave him." I said: "O Messenger of Allah, even if he had cut off one of my hands?" He (peace and blessings be upon him) said: "Even if he did that." I repeated this to him, and he (peace and blessings be upon him) said: "If you kill him after he has said 'There is no true god except Allah', you will be in the position he was in before he said it, and he will be in the position you were in before you killed him."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17718