Maliks Muwatta Book 029, Hadith Number 047.

Section : The Divorce of the Slave.

Yahya related to me from Malik from Abu'z-Zinad from Sulayman ibn Yasar that Nufay, a mukatab of Umm Salama the wife of the Prophet, may Allah bless him and grant him peace, or her slave, had a free woman as a wife. He divorced her twice, and then he wanted to return to her. The wives of the Prophet, may Allah bless him and grant him peace, ordered him to go to Uthman ibn Affan to ask him about it. He found him at ad-Daraj with Zayd ibn Thabit. He asked them, and they both anticipated him and said, "She is haram for you. She is haram for you."