Abdullah bin Buraydah (may Allah have mercy on him) narrates from his father (may Allah be pleased with him) that he said: I was sitting with Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) when suddenly he heard a woman crying out. He said: "O Yarfa', see what this noise is." He went and looked, then came and said: "A girl of Quraysh whose mother is being sold." Sayyiduna Umar (may Allah be pleased with him) said: "Call," or he said: "Call the Muhajirun and the Ansar." In a short while the house and the room were full. Sayyiduna Umar (may Allah be pleased with him) praised and glorified Allah, then said: "Do you know that cutting the ties of kinship was among the things that Muhammad (peace and blessings be upon him) brought?" They said: "No." He said: "Yet it has become widespread among you." Then he recited this verse: "Then would you perhaps, if you turned away, cause corruption on the earth and sever your ties of kinship?" [Surah Muhammad: 22]
Then he said: "What severing of kinship is greater than that the mother of one of you should be sold, when Allah has given you ample means?" The people said: "Decide as you wish." So he had letters written to the regions that the mother of a free man is not to be sold, for that is severing the ties of kinship and it is not permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21773