Muhammad bin Abd al-Qari said: A man came to Sayyiduna Umar (may Allah be pleased with him) from Sayyiduna Abu Musa (may Allah be pleased with him). He asked him about the people, then said: Is there any strange news among you? He said: Yes, a man disbelieved after accepting Islam. He asked: Then what did you do with him? He said: We killed him. Sayyiduna Umar (may Allah be pleased with him) said: Why did you not imprison him for three days, feed him and give him drink, and present Islam to him? Perhaps he would have returned to the command of Allah. O Allah, I was not present, I did not order it, and I am not pleased with it.
Imam al-Shafi'i (may Allah have mercy on him) said in the Book: Whoever says that he is not to be given time claims that the hadith narrated from Sayyiduna Umar (may Allah be pleased with him), "Why did you not imprison him for three days," is not established, because it is not known to be connected; and even if it were established, he did not impose anything on the one who killed him before the three days. The Shaykh (may Allah have mercy on him) said: Another hadith about giving him time has been narrated from Sayyiduna Umar (may Allah be pleased with him) with a connected chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16887
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