Shaikh Muhammad Baazmool (may Allaah preserve him) said in his book titled ‘Al-Muhkam Wal Mutashaabih Fit Takfeer Wal Jihaad’ page 47:

It is Haraam to make takfeer of a Muslim without a Mukaffirah [i.e. a deed of major disbelief he (or she) has committed] or without establishing the proofs. [1]

Full information here: http://salaficentre.com/2015/06/unravelling-doubts-of-the-takfeeris-rebuttal-of-the-aqeedah-of-isis-and-their-supporters-1/

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[1] See here: The Principles of Takfeer

http://www.salafipublications.com/sps/sp.cfm?secID=MNJ&subsecID=MNJ09&loadpage=displaysubsection.cfm