"You shall not bear false witness" — why is it stated? Because it says "You shall do to him as he conspired to do to his brother" (Deuteronomy 19:19): we have heard the punishment, but we have not heard the prohibition. Scripture teaches, saying "You shall not bear false witness."
Do Not Bear False Witness Read as the Prohibition
Yalkut Shimoni on Torah 298:7
לֹא תַעֲנֶה לָמָּה נֶאֱמַר, לְפִי שֶׁהוּא אוֹמֵר (דברים יט, יט) "וַעֲשִׂיתֶם לוֹ כַּאֲשֶׁר זָמַם לַעֲשׂוֹת לְאָחִיו", עֹנֶשׁ שָׁמַעְנוּ אַזְהָרָה לֹא שָׁמַעְנוּ, תַּלְמוּד לוֹמַר לֹא תַעֲנֶה.