Ibn Abbas (may Allah be pleased with them both) said: There were two Jewish tribes, Banu Qurayzah and Banu Nadir. Banu Nadir were more honoured than Banu Qurayzah. If someone of Banu Qurayzah killed (a man of Banu Nadir), he would be killed in retaliation for the man of Banu Nadir, and in the opposite case it was different, that is, 100 wasq of dates would be paid. When the Prophet (peace and blessings be upon him) was sent, a man of Banu Nadir killed a man of Banu Qurayzah. They demanded: We too will kill him in retaliation. The others said: Come, let us have the Prophet (peace and blessings be upon him) judge. They came, and this verse was revealed: "And if you judge, judge between them with justice" [Surah al-Ma'idah: 42], that is, take retaliation (qisas). Then this verse was revealed: "Do they then seek the judgement of the time of ignorance?" [Surah al-Ma'idah: 50]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15880