Musnad Ahmad 33:352

Yahya ibn Adam and Abu al-Nadr narrated to us, they said: Zuhayr narrated to us, Abu al-Zubayr narrated to us, Jabir narrated, he said: Two boys fought, a boy from the Emigrants (Muhajirun) and a boy from the Helpers (Ansar). The Emigrant said: "O for the Emigrants!" And the Helper said: "O for the Helpers!" The Messenger of Allah, peace and blessings of Allah be upon him, came out and said: "Is it the call of Jahiliyyah (pre-Islamic ignorance)?" They said: "No, by Allah, it is only that two boys, one of them kicked the other." He said: "There is no harm. Let a man help his brother, whether he is an oppressor or oppressed. If he is an oppressor, let him forbid him, for that is helping him; and if he is oppressed, let him help him."