—among the other sectarian groups driven by their own whims, such as the Khārijites, for none of them was ever known to lie; on the contrary, people would say, “They are among the most truthful of narrators.” Even an openly immoral man who is notorious for lying must tell the truth on some occasions, for there is no one whose every report is sheer falsehood.

For this reason the Exalted said:
“If a profligate comes to you with a report, verify it” ⦗Q 49:6⦘—
while another canonical reading has: “then ascertain.”

Thus He commanded investigation and verification whenever a sinner conveys news, yet He did not order us to brand him a liar merely because he has spoken, for he may sometimes tell the truth. By enjoining scrutiny and verification of a profligate’s report, the Almighty made it clear that we are not allowed to accept it solely on his say-so—since, as a sinner, he may lie—nor are we allowed to reject it outright until it is established that he has actually lied, because a sinner can also speak the truth.

That is exactly what He said elsewhere:
“O you who believe, when you set out in the path of Allah, investigate” ⦗Q 4:94⦘.