Amir (may Allah have mercy on him) narrates from Qadi Shurayh (may Allah have mercy on him) that along with the claimant's witnesses for his original right, the claimant's oath is (required) for the acquittal of the deceased's family, in case their companion had (already) paid it: 'Billahi alladhi la ilaha illa huwa' (By Allah, besides whom there is no god), he died while this right was still owed by him. And we too hold this in a claim when it is brought against a deceased person, an absent person, a child or an insane person.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21252