Musannaf Ibn Abi Shayba 28:209

'Abd al-Rahim ibn Sulayman narrated to us, from Dawud ibn Abi Hind, who said: 'Amir narrated to us that Anas ibn Malik narrated to him that a group from Bakr ibn Wa'il apostatized from Islam and joined the polytheists, and they were killed in the fighting. When the conquest of Tustar was brought to 'Umar ibn al-Khattab, he said: "What became of the group from Bakr ibn Wa'il?" I said—I diverted to another topic in order to distract him from mentioning them. He said: "What became of the group from Bakr ibn Wa'il?" I said: "They were killed, O Commander of the Faithful." He said: "If I had taken them alive, it would have been more beloved to me than everything upon which the sun rises of gold and silver." I said: "O Commander of the Faithful, what would have been their path if you had taken them, other than killing? A people who apostatized from Islam and joined polytheism." He said: "I would have offered them to enter through the door from which they exited. If they had done so, I would have accepted it from them, and if they had refused, I would have committed them to prison."