Musannaf Ibn Abi Shayba 18:971

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Al-‘Umari narrated to us, from Nafi’, from Ibn ‘Umar: That ‘Umar ibn al-Khattab killed seven men from the people of San’a for one man [whom they had killed]. And he said: "If the people of San’a had participated in it, I would have killed them [all]."