Allah the Exalted ordered investigating the news that sinners and the wicked bring, to make sure of its authenticity. Otherwise, if the sinner's word is taken for granted and a decision is based on it, regardless of whether the information is true or not, the authorities will be taking the lead of the sinners. Allah the Exalted and Most Honored forbade taking the path of the corrupted and sinners. This is why groups of the scholars of Hadith refuse to accept narrations from narrators whose reliability is unknown, for they might be from among the wicked people, in reality.
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