Sahih Muslim Book 001, Hadith Number 0117.

Chapter : The condition of the Faith of one who calls his brother Muslim as an unbeliever.

It is reported on the authority of Ibn 'Umar that the Messenger of Allah (may peace and blessings be upon him) said: Any person who called his brother: or unbeliever (has in fact done an act by which this unbelief) would return to one of them. If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labelled it on his brother Muslim).

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