Al-Aswad (may Allah have mercy on him) narrates from Sayyiduna Umar (may Allah be pleased with him): "Whoever becomes the owner of a relative within the prohibited degrees (dhu mahram), that relative is free."
(b) Another narration says: "A dhu mahram does not remain a slave."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21423