Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) said: We used to sell ummahat al-awlad in the time of the Prophet (peace and blessings be upon him) and of Sayyiduna Abu Bakr (may Allah be pleased with him); then Sayyiduna Umar (may Allah be pleased with him) forbade it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21791