Maymun bin Abi Shabib narrates from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that Sayyiduna Ali (may Allah be pleased with him), when selling a slave woman and her child, separated them, and the Messenger of Allah (peace and blessings be upon him) forbade him from that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18305