Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he said: "If people were given on the basis of their claims, people would start claiming wealth and blood; but the oath is upon the defendant (that is, the one against whom the claim is made)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21197