Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: A slave woman who becomes Umm Walad (the mother of her master's child) becomes free after the death of her master.
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2910
Hadith Gradingحکم دارالسلام:حسن، وهذا إسناد ضعيف، شريك سيئ الحفظ ، لكنه توبع، وحسين بن عبدالله ضعيف