Umar on Adultery: A secret marriage is a type of adultery

Abu Az-Zubair reported: Umar ibn Al-Khattab, may Allah be pleased with him, was presented with a marriage which no one had witnessed except one man and one woman. Umar said, “This is a secret marriage and I do not permit it. If I had known of it beforehand, I would have stoned them.”

Source: al-Muwaṭṭa’ 1136

Ibn Taymiyyah said, “As for a secret marriage, the attestation of which is concealed and witnessed by no one, it is invalid according to the prevalent opinion of scholars. It is a type of illicit intercourse.”

Source: Majmū’ al-Fatāwá 33/158

عَنْ أَبِي الزُّبَيْرِ الْمَكِّيِّ أَنَّ عُمَرَ بْنَ الْخَطَّابِ رضي الله عنه أُتِيَ بِنِكَاحٍ لَمْ يَشْهَدْ عَلَيْهِ إِلَّا رَجُلٌ وَامْرَأَةٌ فَقَالَ هَذَا نِكَاحُ السِّرِّ وَلَا أُجِيزُهُ وَلَوْ كُنْتُ تَقَدَّمْتُ فِيهِ لَرَجَمْتُ

1136  موطأ مالك كتاب النكاح باب جامع ما لا يجوز من النكاح

قال ابن تيمية وَأَمَّا نِكَاحُ السِّرِّ الَّذِي يَتَوَاصَوْنَ بِكِتْمَانِهِ وَلَا يُشْهِدُونَ عَلَيْهِ أَحَدًا فَهُوَ بَاطِلٌ عِنْدَ عَامَّةِ الْعُلَمَاءِ وَهُوَ مِنْ جِنْسِ السِّفَاحِ

33/158 مجموع فتاوى ابن تيمية الفقه الطلاق