Abdullah bin Umar (may Allah be pleased with them both) narrates from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that any slave woman who bears a child to her master is not to be sold, given as a gift or made part of an inheritance; the master may enjoy her, and after his death she is free.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21763