Musnad Abu Ya'la al-Mawsili 142:1

Mansur ibn Abi Muzahim narrated to us, Abu Hafs al-Abbar narrated to us, from Yazid ibn Abi Ziyad, from Mu'awiyah ibn Qurrah, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, had two freedmen, an Abyssinian and a Nabataean. They insulted one another while the Prophet, peace and blessings of Allah be upon him, was listening. One of them said to his companion, "O Abyssinian!" The other replied, "O Nabataean!" The Prophet, peace and blessings of Allah be upon him, said, "Do not say this. You are merely two men from the companions of Muhammad, peace and blessings of Allah be upon him."