Abu al-Zubayr narrates from Sayyiduna Jabir (may Allah be pleased with him) that he said: We used to sell ummahat al-awlad during the lifetime of the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21792