Imam Malik's Muwatta, Chapter: Divorce

Yahya related to me from Malik from Ibn Shihab from Urwa ibn az-Zubayr that a mawla of the tribe of Banu Adi called Zabra told him that she had been the wife of a slave when she was a slave-girl. Then she was set free and she sent a message to Hafsa, the wife of the Prophet, may Allah bless him and grant him peace. Hafsa called her and said, "I will tell you something., but I would prefer that you did not act upon it. You have authority over yourself as long as your husband does not have intercourse with you. If he has intercourse with you, you have no authority at all." Therefore she pronounced her divorce from him three times.