Abu al-Zubayr (may Allah have mercy on him) says that a marriage was brought before Sayyiduna Umar (may Allah be pleased with him) at which only one man and one woman were present (as witnesses). Sayyiduna Umar (may Allah be pleased with him) said: This is a secret marriage, and I do not permit it; had I given a prior warning about it, I would have stoned (them).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13726