Ibn ‘Umar (may Allah be pleased with him) reported that a boy was murdered, and ‘Umar said…

During the time of ‘Umar (may Allah be pleased with him) a group of five or seven people killed a boy inadvertently. ‘Umar (may Allah be pleased with him) ordered that they should all be killed. He did so, confirming the obligation of killing a group of people if they have collaborated in the killing of one person. He said that if all the people of Sanaa had killed one person, he would order them all to be killed. The Companions and jurists have agreed on this ruling, in order that refraining from effectuating retribution would not lead to more collaboration in blood-shedding. ‘Umar (may Allah be pleased with him) mentioned Sanaa in particular, either because the boy’s killers were from there or because the Arabs used to mention the name of Sanaa when referring to huge numbers of inhabitants.