Musnad Ahmad 25:1568

Abu Kamil narrated to us, Nafi’ narrated to us, from Ibn Abi Mulaykah, who said: I wrote to Ibn Abbas, and he wrote back to me: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) said: 'The oath is upon the defendant. If people were given [everything] based on their claims, people would surely claim the wealth of others and their blood.'"