From Malik from Abu'z-Zinad that Umar ibn Abd al-Aziz wrote toAbd al-Hamid ibn Abd ar-Rahman ibn Zayd ibn al-Khattab who was thegovernor of Kufa, "Pronounce judgement on the basis of an oath withone witness."
USC-MSA web (English) reference: Book 36, Hadith 6
Arabic reference: Book 36, Hadith 1410