Ibrahim (may Allah have mercy on him) said: A man from the tribe of Bakr bin Wa'il killed a man from the people of al-Hirah. Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) ruled that the killer be handed over to the heirs of the slain man; if they wished they could kill him, or if they wished they could forgive him. So the man was handed over to one of their heirs, a man named Hunayn, and he killed him. Afterwards Sayyiduna Umar (may Allah be pleased with him) wrote to them: If he has not yet been killed, do not kill him. The people understood that Sayyiduna Umar (may Allah be pleased with him) intended to satisfy them with blood money.
Imam al-Shafi'i (may Allah have mercy on him) said: The view to which he returned is more fitting, and perhaps he intended to frighten him with killing, not to kill him. The one who was debating him said: It has been narrated to you from Amr bin Dinar that Sayyiduna Umar (may Allah be pleased with him) wrote concerning a Muslim who had killed a Christian: If the killer is a habitual killer, then kill him; otherwise leave him and do not kill him. Imam al-Shafi'i (may Allah have mercy on him) said: We have narrated it; so follow Sayyiduna Umar (may Allah be pleased with him) as he said, though you do not follow him in what he said. Then he said: Is anything regarding this established from Sayyiduna Umar (may Allah be pleased with him)? Imam al-Shafi'i (may Allah have mercy on him) said: Not a single letter; the narrations that exist are either munqati' or weak, or both munqati' and weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15928