Musannaf Ibn Abi Shayba 18:746

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Muhammad ibn Qays al-Asadi narrated to us, from ‘Abd al-Malik ibn Maysarah, from al-Nazzal ibn Sabrah, that a man from the Muslims killed a man from the people of al-Hirah. So he wrote regarding him to ‘Umar ibn al-Khattab. ‘Umar wrote: "Execute him for him." It was said to his brother, Hunayn: "Kill him." He said: "Until the anger subsides." He said: It reached ‘Umar that he was one of the cavalrymen of the Muslims. He said: So ‘Umar wrote: "Do not execute him in retaliation for him." He said: The letter reached him while he had already killed him.