Hazrat Sha'bi (may Allah have mercy on him) said: "If a man has one free woman and one slave woman as wives, and he has children from the slave woman, then he buys her, and he dies, the slave woman will not become free, because the children were slaves at the time of their birth."
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 2966
Hadith Takhrij«أخرجه سعيد بن منصور فى «سننه» ترقيم الدرالسلفية برقم:، 1789، والبيهقي فى«سننه الكبير» برقم: 21845»