On the authority of the son of 'Abbas (may Allah be pleased with them both) that the Messenger of Allah* said:
Were people to be given in accordance with their claim, men would claim fortunes and lives of (other) people, but the onus of proof is on claimant and the taking of an oath is incumbent upon him who denies.
A good Hadith related by al-Baihaqi and others in this form, and part of it is in the two Sahihs.
Sahih is the collections of al Bukhari and Muslim.