Imam al-Shafi'i (may Allah have mercy on him) said: "Whoever displays tribalism ('asabiyyah) through his speech, gathers (people) on its basis and calls to it, his testimony is rejected, because he has committed a forbidden act about which, to my knowledge, there is no disagreement among the scholars of the Muslims."
And he cited as evidence the saying of Allah Almighty: "The believers are but brothers." [Surah al-Hujurat: 10]
And the saying of the Messenger of Allah (peace and blessings be upon him): "And be, O servants of Allah, brothers."
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Beware of suspicion, for suspicion is false talk. Do not spy, do not pry, do not try to outdo one another, and do not envy, hate or cut off relations with one another; and, O servants of Allah, be brothers."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21059