Abd al-Rahman bin Abi Layla (may Allah have mercy on him) says that Sayyiduna Ali (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) ordered me to sell two slave boys who were brothers. When selling them I separated them, and I told the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) said: "Find them and take them back, and sell them together; do not separate them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18316