Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: In the time of Jahiliyyah, qasamah was a deterrent among people. Whoever swore a binding oath (yamin sabr) falsely incurred sin by it; he would be shown some punishment from Allah by which those who dared to violate what is forbidden were made an example of. So they would avoid binding oaths and fear them. Then when Allah Almighty sent Muhammad ((peace and blessings be upon him)), he ((peace and blessings be upon him)) retained qasamah, and the Muslims were even more in awe of it because of what Allah Almighty had taught them about it. So the Messenger of Allah (peace and blessings be upon him) judged by qasamah between two clans of the Ansar who were called Banu Harithah. The reason was that the Jews had killed Muhayyisah, but the Jews denied it. So the Messenger of Allah (peace and blessings be upon him) called the Jews to their qasamah, since it was they (Banu Harithah) who were claiming the blood. He ((peace and blessings be upon him)) ordered them that fifty men swear fifty oaths that they were innocent of his killing, but the Jews refused to swear the oaths. Then the Messenger of Allah (peace and blessings be upon him) called Banu Harithah and ordered them that fifty men swear fifty oaths that the Jews had killed him treacherously, and thereby establish their claim against the one whom they alleged had killed their companion; but Banu Harithah also refused to swear the oaths. When the Messenger of Allah (peace and blessings be upon him) saw this, he ruled that the blood money was due from the Jews, because the slain man had been found among them and in their territory.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10691