Maliks Muwatta Book 28, Hadith Number 26.

Section : What is Not Permitted in Marriage in General.

Yahya related to me from Malik from Abu’z-Zubayr al-Makki that a case was brought to Umar about a marriage which had only been witnessed by one man and one woman. He said, “This is a secret marriage and I do not permit it. Had I been the first to come upon it, I would have ordered them to be stoned.”