Musannaf Ibn Abi Shayba 9:856

Ibn Mahdi narrated to us, from Hammad ibn Salamah, from Qatadah, from al-Hasan, Sa’id ibn al-Musayyib, and Khulas, regarding a man who divorced his wife three times while she was pregnant. They said: There is no harm in him marrying her sister during her waiting period. He said: Ubayd ibn Nadlah used to dislike that, until it was mentioned to al-Hasan, and it was as if he refrained from it.