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 isharam for you."
USC-MSA web (English) reference: Book 29, Hadith 48
Arabic reference: Book 29, Hadith 1208