Musannaf Ibn Abi Shayba 9:875

Ibn ‘Uyaynah narrated to us, from ‘Abdullah ibn Abi Yazid, from his father, that Siba‘ ibn Thabit married the daughter of Rabah ibn Wahb. He had a son from other than her, and she had a daughter from other than him. The boy committed fornication with the girl, and pregnancy appeared on the girl. They raised the matter to ‘Umar ibn al-Khattab, and they confessed. He flogged them both and encouraged that they be joined together [in marriage], but the boy refused.