Abdullah bin Dinar said: Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) met a group of riders and asked: "Where have you come from?" They said: "From Sayyiduna Ibn al-Zubayr (may Allah be pleased with him), and he has made lawful for us things that had been forbidden to us." He asked: "What has he made lawful for you?" They said: "Selling ummahat al-awlad." Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: "Do you know Abu Hafs, Sayyiduna Umar (may Allah be pleased with him)?" They said: "Yes." He said: "He forbade selling them, giving them as a gift (hibah) or making them part of an inheritance. The owner may enjoy her as much as he wishes during his lifetime, and after his death she will be free."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21796