Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that any slave woman who becomes an umm walad (the mother of her master's child) becomes free after the death of her master.
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2937
Hadith Gradingحکم دارالسلام:حسن، وهذا إسناد ضعيف ، شريك بن عبدالله النخعي، وحسين بن عبدالله بن عبيدالله بن عباس كلاهما ضعيفان