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
Forty Hadith of an-Nawawi · no. 33