Muwatta Malik 1117

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 ‏.‏

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

Reference

Muwatta Malik 1117

In-book

Book 28, Hadith 27