Episode 51: The onus of proof is upon the claimant
On the authority of Ibn Abbas (may Allah be pleased with him), that the Messenger of Allah (peace and blessings of Allah be upon him) said:
Were people to be given everything that they claimed, men would [unjustly] claim the wealth and lives of [other] people. But, the onus of proof is upon the claimant, and the taking of an oath is upon him who denies.
A hasan hadeeth narrated by al-Baihaqee and others in this form, and part of it is in the two Saheehs.