Sayyiduna al-Miqdad bin al-Aswad (may Allah be pleased with him) said: I said: "O Messenger of Allah, what do you think: if I and a man from the polytheists were to face each other, each of us striking two blows, and he cut off both my hands, then when I lunged at him to strike him, 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: "Even if he had cut off both my hands?" He (peace and blessings be upon him) said: "Even if he did that." I repeated this to him two or three times, and the Prophet (peace and blessings be upon him) said: "If you kill him after he has said 'There is no true god except Allah', then you are like he was before he said it, and he is like you if you kill him."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17717