Musannaf Ibn Abi Shayba 9:361

Ghundar narrated to us, from Ma'mar, from al-Zuhri, from Ubayd Allah ibn Abd Allah ibn Utbah, who said: A man asked Uthman about two sisters being joined together [in marriage]. He said: "A verse permitted them, and a verse prohibited them. I neither command you nor forbid you." He then met Ali at the door, who asked him: "About whom did you ask?" He informed him. Ali said: "But I forbid you. And if I had authority over you, and you then did that, I would have caused you pain."