Musannaf Ibn Abi Shayba 18:742

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Abu al-Ashhab narrated to us, from Abu Nadra, who said: Umar ibn al-Khattab executed a man from the Muslims in retaliation for a man from the people of the dhimma (protected non-Muslims).