Musannaf Ibn Abi Shaybah 29266 — Those who said: When a Muslim kills

Abd al-Malik bin Maysarah said that al-Nazzal bin Sabrah said: "A man from the Muslims killed a Christian inhabitant of al-Hirah. Then a letter was written about this to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), and Sayyiduna Umar wrote: 'Kill him in retaliation for him.' So it was said to the slain man's brother, Hunayn: 'Kill him.' He said: 'Not until anger comes.'" The narrator said: "Then the news reached Sayyiduna Umar (may Allah be pleased with him) that the killer was one of the horsemen of the Muslims, so Sayyiduna Umar wrote again: 'Do not kill him in retaliation.'" The narrator said: "His letter reached them when he had already been killed."