Musannaf Ibn Abi Shayba 9:354

Khalid ibn Makhlad narrated to us, from Malik ibn Anas, from al-Zuhri, from Qabisa ibn Dhu'ayb, who said: Uthman ibn Affan was asked about two sisters from among those whom the right hand possesses, [regarding] combining them [in marriage/intercourse]. He said: "A verse from the Book of Allah made them lawful, and a verse made them unlawful. As for me, I do not like to do that."