1732/1 - Ibn ‘Umar (may Allah be pleased with him and his father) reported that the Prophet (may Allah’s peace and blessings be upon him) said: “If a man says to his fellow Muslim: O disbeliever, it is true about one of them. Either he is truly as he has said, or it will revert to him.” [Narrated by Al-Bukhāri and Muslim]
1733/2 - Abu Dharr (may Allah be pleased with him) reported that he heard the Prophet (may Allah’s peace and blessings be upon him) say: “If a man accuses someone of disbelief or describes him as the enemy of Allah, and he is not like that, this will surely revert to him.” [Narrated by Al-Bukhāri and Muslim]
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1) One should avoid describing Muslims as disbelievers or questioning their religious commitment.
2) One may utter a word displeasing to Allah, and it causes him to lose both his worldly life and afterlife. So, a wise person should think twice before he speaks.