Sayyiduna al-Miqdad bin Amr al-Kindi (may Allah be pleased with him), who fought at Badr, said: I asked the Prophet (peace and blessings be upon him): "I face a man in battle; he cuts off my hand, and then, taking cover behind a tree, he accepts Islam. Should I kill him?" He (peace and blessings be upon him) said: "No." I said again: "O Prophet of Allah! He cut off my hand; should I kill him?" He (peace and blessings be upon him) said: "No. If you kill him, he will attain the position you were in before you killed him, and you will be in the position he was in before he said the word of truth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16823