Yahya related to me from Malik from Abu'z-Zubayr al-Makki that acase was brought to Umar about a marriage which had only beenwitnessed by one man and one woman . He said, "This is a secretmarriage and I do not permit it. Had I been the first to come upon it,I would have ordered them to be stoned."
USC-MSA web (English) reference: Book 28, Hadith 26
Arabic reference: Book 28, Hadith 1120