Shaykhul-Islām ibn Taymiyyah (may Allāh have mercy upon him) mentioned regarding Ḥizbiyyah:

As for the leader of the Ḥizb, he is the head of the group which has become a Ḥizb. If they are united upon that which Allāh has commanded with, as well as His Messenger without any addition or subtraction, then they are believers. For them is that which is for them and upon them is that which is upon them. And if they have indeed added to that or subtracted (from it) – like having Taʿaṣṣub (fanaticism) for whoever enters into their Ḥizb (group/party) based upon the truth and falsehood and turning away from the one who has not entered into their Ḥizb whether he is upon the truth or falsehood – then this is from the separation (into groups and parties) which Allāh and His Messenger have condemned. For indeed Allāh and His Messenger have commanded with the Jamāʿah and the matters of harmony (between the believers) and have forbade the matters of separation and differing. Also Allāh and His Messenger have commanded with cooperation upon righteousness and piety and forbade cooperating upon sin and transgression.

Source: Majmū’ al-Fatāwā 11/92

Translated by Abu Yusuf Khaleefah
18th of Safar, 1437 H (11-30-2015)
Masjid Nur Allah, Queens, NYC

                                    Original Arabic

الحزبية
قال شيخ الإسلام ابن تيمية:
وأما رأس الحزب فإنه رأس الطائفة التي تتحزب؛ أي تصير حزبًا،
فإن كانوا مجتمعين على ما أمر الله به ورسوله من غير زيادة ولا نقصان فهم مؤمنون، لهم ما لهم وعليهم ما عليهم،
وإن كانوا قد زادوا في ذلك ونقصوا؛ مثل التعصب لمن دخل في حزبهم بالحق وبالباطل، والإعراض عمن لم يدخل في حزبهم، سواء كان على الحق أو الباطل، فهذا من التفرق الذي ذمه الله تعالى ورسوله؛
فإن الله ورسوله أمرا بالجماعة والائتلاف، ونهيا عن الفُرقة والاختلاف،
وأمرا بالتعاون على البر والتقوى، ونهيا عن التعاون على الإثم والعدوان.

[ مجموع الفتاوى: 11/ 92]