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.
Reference: 40 Hadith Nawawi 33
English translation: Hadith 33