Ibn Umar (may Allah be pleased with them both) said that Sayyiduna Umar (may Allah be pleased with him) forbade the sale of ummahat al-awlad (slave women who have borne their masters' children). He (may Allah be pleased with him) said: She is not to be sold, nor given as a gift, nor made part of an inheritance. Her master may enjoy her, and when he dies she becomes free.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21764