It is narrated in Sonan Abu Dawood: In the Battle of Uhud one Farsi (Persian/Iranian) slave was among Muslims and in one attack beat his opponent and then proudly said: I am a Farsi youth. The Prophet (PBUH) came forward and told him: Never say that, instead of that; say: I am a youth from Ansar; why? Because the holy Prophet (PBUH) knew the moment he said: I am a Farsi youth, another one would say: I am an Arab youth, and this will cause disagreement and disunity among Islamic Ummah (Islamic nation) [1].
[1] Islam va Mass’aleye melliyyat, Martyr Morteza Motahhari (RA), Page 42.