Hadith: Ibn ‘Umar (may Allah be pleased with him) reported that a boy was murdered, and ‘Umar said: If all the people of Sanaa took part in it, I would kill them all

عن ابن عمر رضي الله عنهما أنَّ غلامًا قُتل غِيلَةً، فقال عمر رضي الله عنه : «لو اشْتَرَكَ فيها أهْلُ صَنْعاء لَقَتَلْتُهم».
[صحيح] - [رواه البخاري]

Ibn ‘Umar (may Allah be pleased with him) reported that a boy was murdered, so ‘Umar said: "If all the people of Sanaa took part in murder I would kill them all."
[Authentic hadith] - [Narrated by Bukhari]

Explanation

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.