Abu al-Zubayr heard Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) saying about the children of a mudabbarah that after the master's death they are free. (b) Ata (may Allah have mercy on him) says that the children of a mudabbarah are slaves, unless she was pregnant on the day she was made mudabbarah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21595