Shaykhul-Islam Ibn Taymiyyah (may Allāh have mercy upon him) stated:

Whoever has fanaticism towards his land, madh-hab, way (i.e. methodology), family members, and friends, to the exclusion of anything else, then he has within himself a branch from the (branches of) Jāhiliyyah (pre-Islamic era of ignorance). (This fanaticism is prohibited) so that the believers behave as Allah has commanded them to. That they are all to hold on to His Rope, His Book, and the Sunnah of His Messenger together. For indeed their (i.e. the believer’s) book is one; their religion is one; their Prophet is one; their Lord is one, none having the right to be worshiped except He. For Him is the praise in the beginning and the end. For Him is the rulership. And to Him you all shall return.

Source: Al-Fatāwā, 28/422-423

Translated by Abu Yusuf Khaleefah
2nd of Ṣafar, 1437 AH (11/15/2015)
Masjid Nur Allāh, Queens, NYC

Original Arabic

بسم الله الرحمن الرحيم

قال شيخ الإسلام ابن تيمية رحمه الله:

فمن تعصب لأهل بلدته, أو مذهبه, أو طريقته, أو قرابته, أو لأصدقائه دون غيرهم, كانت فيه شعبة من الجاهلية, حتى يكون المؤمنون كما أمرهم الله تعالى معتصمين بحبله وكتابه وسنة رسوله, فإن كتابهم واحد, ودينهم واحد, ونبيهم واحد, وربهم إله واحد لا إله إلا هو, له الحمد في الأولى والآخرة, وله الحكم, وإليه ترجعون

الفتاوى: 28/ 422, 423