Musannaf Ibn Abi Shaybah

Hazrat Hakam narrates that Hazrat Ali (may Allah be pleased with him) was asked about a man who divorced a woman and, during her waiting period (‘iddah), married her sister. What is the ruling regarding this? Hazrat Ali (may Allah be pleased with him) separated the two and made the dowry (mahr) obligatory upon the man. And he said that when the waiting period of the first wife is completed, he may send a marriage proposal; if he has consummated the marriage, then the full dowry will be obligatory, and the woman will have to complete the full waiting period, and both women will observe the waiting period, and the waiting period for each will be three menstrual cycles. If they do not menstruate, then they will observe the waiting period for three months.