Sayyiduna Anas bin Malik (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not hate, envy or cut off relations with one another; be brothers among yourselves. No Muslim should cut off his brother for more than three days, such that when the two meet, each turns away from the other; and the better of them is the one who begins with the greeting of peace."
Imam al-Shafi'i (may Allah have mercy on him) said: Allah has gathered people through Islam and ascribed them to it, so it is the noblest of their lineages; so if a man loves, let him love on that basis.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21061