Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 180
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 180, with English translation by Maimonides (Rambam).
לעשות לעדים זוממין כמו שזמו לעשות, שנאמר: "ועשיתם לו, כאשר זמם" (דברים יט,יט).
To [punish] false witnesses [by] giving them the same penalty that they wish to have imposed [on the defendant], as [Deuteronomy 19:19] states: "And you shall do to him what he plotted to do to his brother."