Sayyiduna Miqdad bin al-Aswad (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him): If I fight a disbeliever and he cuts off my hand with a sword, and then, escaping from me, he recites the kalimah and hides behind a tree, shall I kill him even though he has accepted Islam? He (peace and blessings be upon him) said: "Do not kill him." I said: O Messenger of Allah! He cut off my hand; shall I then kill him? He said: "Do not kill him; for if you kill him, he will be in the position you were in before you killed him, and you will be in the position he was in before he said the word he said."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15846