It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he separated a slave woman and her child (when selling them), so the Prophet (peace and blessings be upon him) forbade him from that and annulled the sale (transaction).

It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that on the day of Hudaybiyah, before the peace treaty, some slaves fled and came to the Messenger of Allah (peace and blessings be upon him). Their masters wrote to him (peace and blessings be upon him) and said: O Muhammad, by Allah, these people have not come to you out of desire for your religion; rather, they have come fleeing from slavery. Some people said: O Messenger of Allah, they have spoken the truth; return them (the slaves) to them. At this, the Messenger of Allah (peace and blessings be upon him) became angry and said: "O people of Quraysh, I think you will not desist until Allah sends against you someone who will strike your necks over this matter." And he (peace and blessings be upon him) refused to return them, and said: "They are the freed ones of Allah." This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.