Musannaf Ibn Abi Shayba 9:236

'Abdah ibn Sulayman narrated to us, from Sa'id, from Qatadah, from al-Hasan, and from Abu Ma'shar, from Ibrahim, regarding the slave woman who is under a slave: he divorces her two pronouncements, then they are both emancipated. They both said: She is not lawful for him until she marries a husband other than him.