On the authority of Ibn Abbas that the Messenger of Allah (Sallallaahu 'alaihi wasallam), said:
"Were people to be given in accordance with their claim, men would claim the fortunes and lives of [other] people, but the onus of proof is on the claimant, and the taking of an oath is incumbent upon him who denies."
A Hasan (fair) Hadith related by Al-Baihaqi and others