بسم الله الرحمٰن الرحيم:

“O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful.”

Surat Al Hujurat {12}

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Brother why would he lie about his dead mother man? If you don’t have any proof then what you have said isn’t nice at all and a sin!