Shaikh Zaid Bin Haadi Al-Madkhali (rahimahullaah) said:

It is not permissible for the one who knows something about the innovation of the partisan group Ikhwaan Al-Muslimeen (i.e. The Muslim Brother’-a deviated sect founded by the Soofee Hasan Al-Bannah of Eyypt) or other than them amongst the people of innovation and error that he remains silent about clarifying their affair, even though he has the ability to clarify. That is because refuting the people of innovation and error is obligatory upon those who have the ability to do so amongst the People of Knowledge.   And when (refutation) is established by some of them the obligation is lifted from the rest.

And as for fearing splitting as a result of refutation against the sects, there is no legislated excuse for refraining (i.e. refraining from carrying out such refutation). That is because having love for Ijtimaa (unity upon the sound creed and methodology of the Salaf) is the correct stance and is more deserving to be desired and followed.  The Mubtadi (the innovator) is the cause of splitting in the Ummah because splitting is connected to innovation and unity is connected to the Sunnah.

And the obligation of refutation against the Mukhaalif (i.e. the one in opposition to the truth) is not lifted from the scholar due to anticipation of harm, unless it is harm that he is not able to bear (or repel); then (in this case), Allah does not burden a soul beyond what it can bear.  The earth is not devoid of people of knowledge who will carry out refutation against the innovator and the Mukhaalif.

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[Source: Al Ajwibah Al Mukhtasar Alaa  As’ila Al’Ashrati’ Pages 43-44]