Imam al-Bukhari (may Allah have mercy on him) said in the chapter heading: Ibn Bashshar narrated to me, from Yahya, from Ubaydullah, from Nafi', from Sayyiduna Ibn Umar (may Allah be pleased with him), that a boy was killed by treachery, and Sayyiduna Umar (may Allah be pleased with him) said: "If the people of Sana'a had taken part in killing him, I would have killed them all."
It is also reported from Yahya bin Sa'id that a boy was killed by treachery in Sana'a, so Sayyiduna Umar (may Allah be pleased with him) executed seven people for him and said: "If all of Sana'a had taken part in killing him, I would have killed them all."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15974