Abd al-Salam bin Harb narrates something similar: that he separated a slave woman and her child, and the Prophet (peace and blessings be upon him) "forbade him and annulled the sale."
Abu Dawud said: Maymun did not meet Ali (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18306