Musnad Ahmad 5:481

'Abd al-Wahhab narrated to us, from Sa'id, from a man, from al-Hakam ibn 'Utaybah, from 'Abd al-Rahman ibn Abi Layla, from 'Ali, that he said: The Messenger of Allah, peace and blessings of Allah be upon him, commanded me to sell two young slave brothers. So I sold them, and I separated them. I mentioned that to the Prophet, peace and blessings of Allah be upon him, and he said: "Overtake them and take them back, and do not sell them except together, and do not separate them."