THE FATWAA OF SHAYKH IBNUL-’UTHAYMEEN:
[Q]: Are there any texts from the Book of Allaah, or from the Sunnah of His Messenger (sallallaahu ’alayhi wa sallam) which contain an allowance of multiple groups and parties or al-Ikhwaan?
[A]: I say: There is not either in the Book or the Sunnah that which allows multiple groups and parties. Rather, there is in the Book and the Sunnah that which censures this. Allaah - the Most High - said:
"Verily those who split up their Religion and become sects, you have no concern with them in the least. Their affair is with Allaah who will tell them about what they used to do." [Sooratul An’aam 6:159]
Allaah - the Most High - said:
"Each sect rejoicing in that which is with itself." [Sooratur-Room 30:32]
And there is no doubt that these parties are contrary to what Allaah ordered. Rather what Allaah has ordered and encouraged is in His saying:
"And verily! This your religion (of Islâmic Monotheism) is one religion, and I am your Lord, so keep your duty to Me. (Al-Mu'minun 23:52)" [Sooratul-Mu‘minoon 23:52]
And some people say: It s not possible for the da’wah (call) to be strong unless it is done within the framework of a party! We say to that: This is incorrect! Rather the da’wah will be strong whenever the person is under the Book of Allaah and the Sunnah of His Messenger (sallallaahu ’alayhi wa sallam) - following the narrations of the Prophet (sallallaahu ’alayhi wa sallam) and that of his Companions.
Source: www.troid.org (with Qur'an translations taken from thenoblequran.com)
