Ruzayq bin Hakim (may Allah have mercy on him) said: I wrote to Umar bin Abd al-Aziz (may Allah have mercy on him) that (judging by) an oath along with a witness is found only in Madinah. He wrote to me: Judge in this way, for it is the Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20680