Musannaf Ibn Abi Shayba 9:494

Abu Bakr narrated to us, he said: Hushaym informed us, from Yunus, from al-Hasan, that a man married a woman and kept it secret, and he used to visit her at her house. A neighbor of hers saw him entering upon her and accused her of adultery. He disputed with him before Umar ibn al-Khattab. He said: "O Commander of the Faithful, this man used to enter upon a young woman, and I do not know that he married her." He (Umar) said to him (the husband): "What do you say?" He said: "I married a woman for something small, and I kept that secret." He (Umar) said: "Who witnessed it for you?" He said: "I had some of her family witness it." He (Umar) warded off the prescribed punishment from the one who accused her, and he said: "Publicize this marriage and protect these private parts."