אל תשת ידך עם רשע להיות עד חמס
Don not extend your hand with the wicked to be a false witness.
The world of Chassidus repeatedly emphasizes the concept that the Mitzvos that pertain to worldly affairs are physical manifestation of spiritual concepts. (This fundamental principle will be explained more fully in an upcoming article). We are challenged to feel the deeper significance of the practical Mitzvos that we do.
A Jewish court of law only considers a pair of witnesses a source of valid testimony. Here we are commanded not to conspire with someone and bear false witness. R' Moshe of Pshevarsk, zy"a explains that one doesn't necessarily need a court of law or a human cohort to violate the spirit of this prohibition.
We know that there is a Prosecutor, the Satan (who alternatively plays the roles of Angel of Death and Evil Inclination) who speaks before the Heavenly Court and attempts to elicit verdicts that will harm the Jewish people. He will slander a particular Jew in order to bring down upon him some form of discomfort. Thankfully, the Court upstairs plays by the same rules as the ones down here; thus the Prosecutor is only a single witness and is not believed on his own merit.
However, when a person speaks negatively about another Yid the prosecuting angel has found his partner-in-crime. His testimony coupled with the Lashon HaRa of a person in this world can sway the Heavenly court as any regular pair of witnesses would.
We think that talk is cheap, but here we learn that a Jew's power of speech is so potent that it dovetails with the words of angels in a way that can have very real consequences.
Our Pasuk alludes to this idea. "אל תשת ידך עם רשע להיות עד חמס". The 'wicked one' being referred to here is the Satan. The Torah is instructing us not to dare twin our testimony with his, for doing so can peoduce dangerous results.