Maliks Muwatta Book 29, Hadith Number 48.

Section : The Divorce of the Slave.

Yahya related to me from Malik from Ibn Shihab from Said ibn al-Musayyab that Nufay, a mukatab of Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, divorced his free wife twice, so he asked Uthman ibn Affan for an opinion, and he said, “She is haram for you.”