It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that a slave of hers divorced his free wife with two divorces. So Sayyidah Umm Salamah (may Allah be pleased with her) asked the Prophet (peace and blessings be upon him) for a ruling about it, and the Messenger of Allah (peace and blessings be upon him) said: "She has become unlawful for him until she marries another husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19944