Musnad Ahmad 30:3836

Muhammad ibn Mus'ab narrated to us, al-Awza'i narrated to us, from al-Zuhri, from Sa'id, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, entered the mosque while the Abyssinians were playing. Umar rebuked them, but the Prophet, peace and blessings of Allah be upon him, said: "Leave them, O Umar, for they are the sons of Arfida."