Musnad Abu Ya'la al-Mawsili 110:163

Abu Bakr narrated to us, Mus'ab ibn al-Miqdam narrated to us, from Mandil, from Ibn Jurayj, from 'Amr ibn 'Ata, from 'Ikrimah, from Ibn 'Abbas, who said: A female servant of the family of the Messenger of Allah (ﷺ) committed adultery. He (the Prophet) said: "O Ali, punish her." He said: So he left her until she delivered what was in her womb, then he struck her fifty [times]. Then he came to the Messenger of Allah (ﷺ) and mentioned that to him, and he said: "You have done correctly."