Sa'id bin al-Musayyib (may Allah have mercy on him) said that Sayyiduna Umar (may Allah be pleased with him) freed the ummahat al-awlad. He said: "The Messenger of Allah (peace and blessings be upon him) had also freed them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21772