Abu Dawud
The Office Of The Judge
Book Number: 018 / Hadith Number: 3584
Narated By Abdullah ibn Abbas : When this verse was revealed: "If they do come to thee, either judge between them, or decline to interfere....If thou judge, judge in equity between them." Banu an-Nadir used to pay half blood-money if they killed any-one from Banu Qurayzah. When Banu Qurayzah killed anyone from Banu an-Nadir, they would pay full blood-money. So the Apostle of Allah (pbuh) made it equal between them.