Sayyiduna Usamah bin Zayd (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) would hold me and Hasan (may Allah be pleased with him) and say: "O Allah, I love them both, so love them both too."
Imam al-Shafi'i (may Allah have mercy on him) said: What is disliked in a man's love for those who belong to him is that he imposes on others what Allah has forbidden him: transgression, attacking lineage, and tribalism and hatred on the basis of lineage, not on account of disobedience to Allah nor any wrong done by the hated person to the one who hates him, but rather he says: I hate him because he is from Banu so-and-so. This is pure tribalism, for which testimony is rejected.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21074