It is narrated from Sayyiduna Ali (may Allah be pleased with him): "The oath goes with the witness. If he has no proof, then the oath is upon the one against whom the claim is made, provided he has had dealings with him. If he declines, the claimant takes the oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20741