Musnad Ahmad 5:198

Muhammad ibn Ja‘far narrated to us, Sa‘id—meaning Ibn Abi ‘Arubah—narrated to us, from al-Hakam ibn ‘Utaybah, from ‘Abd al-Rahman ibn Abi Layla, from ‘Ali ibn Abi Talib, who said: The Messenger of Allah (may Allah bless him and grant him peace) commanded me to sell two young slave brothers. I sold them and separated them. I mentioned that to the Prophet (may Allah bless him and grant him peace), and he said: "Overtake them and bring them back, and do not sell them except together."