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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / Q21059