Musannaf Ibn Abi Shayba 9:362

'Abd al-A'la narrated to us, from Ibn Ishaq, from 'Asim ibn 'Umar, from Qatada, from al-Qasim ibn Muhammad, that a tribe from the tribes of the Arabs asked Mu'awiya about two sisters from what the right hand possesses, who are with a man, and he has intercourse with them. He said: "There is no harm in that." Al-Nu'man ibn Bashir heard of that and said: "You gave a verdict of such and such?" He said: "Yes." He said: "Have you considered if a man had his sister as a slave, would it be permissible for him to have intercourse with them?" He said: "As for by Allah, you have indeed cornered me. Go and tell them to avoid that, for it is not appropriate for them." He [the narrator] said: I said: "It is only the kinship of emancipation and other than it."