Musannaf Ibn Abi Shayba 9:838

Jarir narrated to us, from Mughira, from Hammad, from Ibrahim, who said: If a man marries a woman, then divorces her, then marries her sister while she is in her waiting period (iddah), he said: Her marriage is forbidden, and they are to be separated. She has no dowry, and she has no waiting period (to observe).