Muwatta Malik, Book 28, Hadith 1120 (In-Book) ⓘ

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي الزُّبَيْرِ الْمَكِّيِّ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، أُتِيَ بِنِكَاحٍ لَمْ يَشْهَدْ عَلَيْهِ إِلاَّ رَجُلٌ وَامْرَأَةٌ فَقَالَ هَذَا نِكَاحُ السِّرِّ وَلاَ أُجِيزُهُ وَلَوْ كُنْتُ تَقَدَّمْتُ فِيهِ لَرَجَمْتُ ‏.‏