Ibn Abi Mulaykah narrates that a slave woman was given a kitabah contract, then she gave birth to two children, then she died. I asked Abdullah bin al-Zubayr (may Allah be pleased with him) about her, and he said: "If the two of them take up their mother's kitabah, that is their right, and if they pay it off, they are freed."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21703