Musannaf Ibn Abi Shayba 9:851

Hafs narrated to us, from Ash'ath, from al-Hakam, from Ali, that he was asked about a man who divorced his wife, and her waiting period (iddah) had not yet expired when he married her sister. Ali separated them and assigned her the dowry (sadaq) for what he had made lawful of her private parts. He said: "If he had consummated the marriage with her, she is entitled to the full dowry, and she must observe the full waiting period. Both of them shall observe the waiting period from him, each of them for three menstrual cycles. If they do not menstruate, then three months."