Musannaf Ibn Abi Shayba 28:226

Ibn ‘Uyaynah narrated to us, from Muhammad ibn ‘Abd al-Rahman, from his father, who said: When the conquest of Tustar—and Tustar is from the land of Basra—arrived to ‘Umar, he asked them, "Is there any news from the west?" They said, "A man from the Muslims joined the polytheists, so we captured him." He said, "What did you do with him?" They said, "We killed him." He said, "Would that you had brought him in, locked a door upon him, fed him a loaf of bread every day, and then sought his repentance for three days. If he repented, [you would have released him], and if not, you would have killed him." Then he said, "O Allah, I did not witness it, I did not command it, and I was not pleased with it when it reached me," or he said, "at the time it reached me."