Musannaf Ibn Abi Shayba 9:825

Abu Usama narrated to us, from Umar ibn Hamza, who said: I heard al-Qasim ibn Muhammad saying: Ibn al-Zubayr judged among us regarding a freedwoman of theirs whose husband had been announced dead, so she remarried. Then her husband came, and he (Ibn al-Zubayr) judged that her first husband is to be given the choice: if he wishes, he may have his wife, and if he wishes, he may have his dowry. Umar said: And al-Qasim used to say that.