Musannaf Ibn Abi Shayba 18:739

Abu Bakr narrated to us, he said: Ali bin Mushir narrated to us, from al-Shaybani, from Abd al-Malik bin Maysarah, from al-Nazzal bin Sabrah, who said: A man from the cavalry of the people of Kufa killed an 'Ibadi (a Christian) from the people of al-Hirah. So Umar wrote that they should retaliate against his brother for him. They handed the man over to the brother of the 'Ibadi, and he killed him. Then, Umar’s letter arrived [saying] that they should not kill him, but he had already killed him.