Imam Malik's Muwatta, Chapter: Marriage

Yahya related to me from Malik from Ibn Shihab from Qabisa ibn Dhu'ayb that a man asked Uthman ibn Affan whether one could have intercourse with two sisters who one owned. Uthman said, "One ayat makes them halal, and one ayat makes them haram. As for me, I wouldn't like to do it." The man left him and met one of the companions of the Messenger of Allah, may Allah bless him and grant him peace, and asked him about it, and he said, "Had I any authority and I found someone who had done it, I would punish him as an example." Ibn Shihab added, "I think that it was Ali ibn Abi Talib."