Muwatta Malik #1184

Yahya related to me from Malik that he had heard that az-Zubayr ibn al-Awwam said the like of that. Malik said that if a man had sexual relations with a female slave that he owned, and then he wanted to also have relations with her sister, the sister was not halal for a man until intercourse with the slave-girl had been made haram for him by marriage, setting free, kitaba, or the like of that - for instance, if he had married her to his slave or someone other than his slave.