Musnad Ahmad 25:193

Yahya narrated to us, from Ali ibn al-Mubarak, who said: Yahya ibn Abi Kathir narrated to me, that Umar ibn Mu'tab informed him, that Abu Hasan, the freed slave of Abu Nawfal, informed him that he asked Ibn Abbas for a legal opinion regarding a male slave who had a female slave under him, and he divorced her twice, then they were both emancipated. Is it permissible for him to propose to her? He said: "Yes, the Messenger of Allah, may Allah bless him and grant him peace, decreed that."