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]

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[1] See here: The Principles of Takfeer