Musannaf Ibn Abi Shayba 9:355

Abu Mu'awiyah narrated to us, from Hajjaj, from Maymun, that Ibn Umar was asked about a man who has two slave-girls who are sisters; may he have intercourse with the other if he has had intercourse with one of them? He said: Ibn Umar said, "He may not have intercourse with the other as long as the one he has had intercourse with is in his possession."