Maliks Muwatta Book 029, Hadith Number 049.

Section : The Divorce of the Slave.

Yahya related to me from Malik from Abdu Rabbih ibn Said from Muhammad ibn Ibrahim ibn al-Harith at-Taymi that Nufay, a mukatab of Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, asked Zayd ibn Thabit for an opinion. He said, "I have divorced my free wife twice." Zayd ibn Thabit said, "She is haram for you."