Qatadah (may Allah have mercy on him) reported that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), Umar bin Abd al-Aziz (may Allah have mercy on him) and the caliphs between them used to set the ummahat al-awlad free.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21776