Musnad Abu Ya'la al-Mawsili 110:269

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Nafi‘ ibn ‘Umar, from Ibn Abi Mulaykah, who said: Ibn Abbas wrote to me that the Messenger of Allah (may Allah bless him and grant him peace) said: "If people were given based on their claims, then some people would claim the blood of others and their wealth. But the oath is upon the defendant."