Musnad Ahmad 5:258

'Affan narrated to us, Hammad ibn Salamah narrated to us, al-Hajjaj informed us, from al-Hasan ibn Sa'd, from his father, that Yuhannis and Safiyyah were among the captives of the khums (one-fifth share). Safiyyah committed adultery with a man from the khums, and she gave birth to a boy. The adulterer and Yuhannis both claimed him. They disputed before 'Uthman ibn 'Affan, so he referred them to 'Ali ibn Abi Talib. 'Ali said: "I will judge regarding this with the judgment of the Messenger of Allah, peace and blessings of Allah be upon him: 'The child belongs to the bed, and for the adulterer is the stone.'" And he flogged them both fifty [lashes] each.