Al-Hasan (may Allah have mercy on him) says that a man secretly married a woman and used to visit her. A neighbour of hers saw him and accused him with her, and the matter was taken to Sayyiduna Umar (may Allah be pleased with him). Sayyiduna Umar (may Allah be pleased with him) said: Who is your witness to your marriage to her? He said: O Commander of the Believers! It was a private matter, or a small affair, and I made her family witnesses to it. So Sayyiduna Umar (may Allah be pleased with him) did not impose the prescribed punishment (hadd) on the one who had accused him, and said: Safeguard the chastity of these women and announce marriage; and he forbade mut'ah (temporary marriage).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14696