Praise be to Allaah.

The way to unite the Muslims on the truth and do away with division and disputes is to adhere to the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him), and to persist in that and to advise one another to do so, and to cooperate in righteousness and piety, and to refer all disputed matters to the Book of Allaah, may He be glorified, and the Sunnah of His Messenger (peace and blessings of Allaah be upon him), and to refer to them for judgement in all things, as Allaah says (interpretation of the meaning): 

“O you who believe! Obey Allaah and obey the Messenger (Muhammad صلى الله عليه وسلم), and those of you (Muslims) who are in authority. (And) if you differ in anything amongst yourselves, refer it to Allaah and His Messenger (صلى الله عليه وسلم), if you believe in Allaah and in the Last Day. That is better and more suitable for final determination”

[al-Nisa’ 4:59] 

“And in whatsoever you differ, the decision thereof is with Allaah (He is the ruling Judge)”

[al-Shoora 42:10]. 

“those of you (Muslims) who are in authority” are the scholars who have knowledge of the religion of Allaah and are known for good ‘aqeedah (belief) and conduct, and the Muslim rulers. If a dispute arises among them concerning any matter, it must be referred to Allaah and the Messenger (peace and blessings of Allaah be upon him). Referring to Allaah means referring to the Holy Qur’aan, and referring to the Messenger (peace and blessings of Allaah be upon him) means referring to him during his lifetime and to his saheeh Sunnah after his death. What they or one of them rules is the ruling of Allaah, may He be glorified and exalted. What all Muslims, governments and people, scholars and rulers, must do is fear Allaah by obeying His commands and heeding His prohibitions, and referring to the Book of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him) in matters concerning which they dispute among themselves, in obedience to the two aayahs quoted above, and in accordance with the words of Allaah (interpretation of the meaning):  

“But no, by your Lord, they can have no Faith, until they make you (O Muhammad صلى الله عليه وسلم) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission”

[al-Nisa’ 4:65] 

“By Al‑‘Asr (the time).

2. Verily, man is in loss,

3. Except those who believe (in Islamic Monotheism) and do righteous good deeds, and recommend one another to the truth [i.e. order one another to perform all kinds of good deeds (Al‑Ma‘roof) which Allaah has ordained, and abstain from all kinds of sins and evil deeds (Al‑Munkar) which Allaah has forbidden], and recommend one another to patience (for the sufferings, harms, and injuries which one may encounter in Allaah’s Cause during preaching His religion of Islamic Monotheism or Jihad)”

[al-‘Asr 103:1-3] 

“And hold fast, all of you together, to the Rope of Allaah (i.e. this Qur’aan), and be not divided among yourselves”

[Aal ‘Imraan 3:103] 

We ask Allaah by His most beautiful names and by His sublime attributes to set straight the affairs of all the Muslims in all places, and to soften their hearts towards the truth, and to unite them in true guidance, and to grant them refuge from the tricks of the shaytaan and the plots of their enemies, and to guide their leaders and to appoint the best of them in authority over them, for He is All Hearing, Ever Near. End quote. 

And Allaah knows best. 

Majmoo’ Fataawa Ibn Baaz (27/359).

See also the answer to question no. 12110.