Musannaf Ibn Abi Shayba 36:227

Khalid ibn Makhlad narrated to us, he said: Ibrahim ibn Isma'il narrated to us, he said: 'Abd al-Rahman ibn Thabit and Dawud ibn al-Husayn informed me, from a Persian man, a freed slave of Banu Mu'awiyah, that he struck a man on the day of Uhud and killed him, and said: "Take this, and I am the Persian youth." The Messenger of Allah (peace and blessings of Allah be upon him) said: "What prevented you from saying 'the Ansari,' for you are one of them? Indeed, the freed slave of a people is one of them."