It is narrated from Sayyiduna Ali (may Allah be pleased with him) that some captives were brought to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) ordered me to sell two brothers. I sold them and separated them from each other. Then I came to the Prophet (peace and blessings be upon him) and informed him, and he (peace and blessings be upon him) said: "Go after them and bring them back, and sell them both together; never separate them from each other." This hadith is gharib (unusual) and sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it. According to one report, this narration is from al-Hakam, through Maymun bin Abi Shabib, from Sayyiduna Ali (may Allah be pleased with him), and that too is sahih:

It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that he sold a slave woman and her child and separated them from each other, so the Messenger of Allah (peace and blessings be upon him) forbade him from doing that. This is a sahih (authentic) text with another chain.

It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Accursed is the one who causes separation (between a mother and her child)." This chain is sahih (authentic), but the two Shaykhs (Bukhari and Muslim) did not record it, and its explanation is found in this hadith of Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him):

It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever causes separation between a mother and her child, Allah Almighty will cause separation between him and his loved ones on the Day of Resurrection." This hadith is sahih (authentic) on the condition of Muslim, and the two Shaykhs (Bukhari and Muslim) did not record it.