Hanash said that Sayyiduna Ali (may Allah be pleased with him) used to require witnesses along with the oath as well.
(b) Hanash narrates from Sayyiduna Ali (may Allah be pleased with him) that Sayyiduna Ali (may Allah be pleased with him) held this view when the proofs conflicted.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21248
Ibn Sirin (may Allah have mercy on him) said that a man claimed a right against someone and also presented a witness. Qadi Shurayh asked him to swear an oath, and it was as if he was refusing the oath. So the judge said: What a poor commendation you are giving your witnesses!
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21249
Abu Malik al-Ashja'i (may Allah have mercy on him) said: I came to Qadi Shurayh (may Allah have mercy on him) when two men came with a dispute over a riding animal, (one claiming) that the animal had given birth in his care. Qadi Shurayh (may Allah have mercy on him) asked for witnesses. He brought a group of eight people as witnesses. The one in whose possession the animal was said: Ask him to swear an oath. So the judge asked him to swear an oath. He said: Look, I have presented eight witnesses! Qadi Shurayh (may Allah have mercy on him) said: Present as many witnesses as you like; as long as you do not swear the oath yourself, judgement will not be given in your favour.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21250
Awn bin Abdullah bin Utbah (may Allah have mercy on him) narrates from his father that he asked a man to swear an oath along with his witness, and he refused to swear. So Abdullah bin Utbah (may Allah have mercy on him) said: I will not give judgement for you for wealth until you swear an oath.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21251
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