Sayyiduna Anas (may Allah be pleased with him) said: When we camped at Tustar; then he mentioned the long hadith about the conquest and said: We came to Sayyiduna Umar (may Allah be pleased with him) and he asked: O Anas, what became of those six men of Bakr bin Wa'il who apostatised? He said: I wanted to occupy him with another matter, but he asked the same question again, so I said: They were killed in the battle. He said: "Inna lillahi wa inna ilayhi raji'un (We belong to Allah and to Him we return)." I said: O Commander of the Believers, was there any other way for them? He said: Yes, I would have presented Islam to them so that they might accept it; if they refused, I would have put them in prison.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16888