A person earns credibility by habituating himself to always speak the truth. Such a person has earned a reputation of faithful adherence to the truth. In contrast a lying witness will be found out and punished accordingly.
כח = עֵד כְּזָבִים יֹאבֵד וְאִישׁ שׁוֹמֵעַ לָנֶצַח יְדַבֵּר
(28) A lying witness will perish
but a man who listens will speak his testimony forever.
The proverb contrasts the credibility of a person who obeys the Commandment against bearing false witness (Shemos 20:13) with the bitter end of the witness who testifies falsely.
לֹא תַעֲנֶה בְרֵעֲךָ עֵד שָׁקֶר
“You shall not bear false witness against your fellow”
A lying witness — עֵד כְּזָבִים
will perish — יֹאבֵד
but a listening man — וְאִישׁ שׁוֹמֵעַ
(who is known to observe the Commandment
forbidding the bearing of false witness)
will be permitted to testify forever — לָנֶצַח יְדַבֵּר.
 The death verdict for the false witness is understood to be carried out by the Heavenly Court. (אוצר פירושים)
 In an alternate interpretation, the “listening man” refers to all the people who unknowingly hear and repeat the false words of the lying witness. Even though the witness will perish, the consequences of his slander will persist for generations to come. (מצודות)
 We can draw an important lesson from the fact that even after the original false witness is dead and buried, his slanderous testimony may be heard and repeated by untold future generations. (אלשיך)
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