Sa'id bin al-Musayyib (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) ordered: "The ummahat al-awlad are to be set free, they are not to be counted within the third (of the estate), and they are not to be sold to pay debts."
Ja'far said: No one other than him (Abd al-Rahman bin Ziyad) narrated this hadith.
(b) Sa'id bin al-Musayyib (may Allah have mercy on him) said concerning the ummahat al-awlad: People say that the first ruling about the ummahat al-awlad, that they be set free, was from Sayyiduna Umar (may Allah be pleased with him), but that was not so. The first to free them was the Prophet (peace and blessings be upon him); he did not count them within the third, and did not sell them for debts.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21771