Sifrei Devarim 189

¹(Devarim 19:16) "If there arise a witness chamas against a man to testify against him sarah ": " chamas " connotes a robber, and " sarah ," as in ( Ibid . 13:6) "for he has spoken sarah ," and (Jeremiah 28:16) "This year you shall die, for you have spoken sarah of the L-rd." ²We are hereby taught that he is not liable until he falsifies himself — whence they ruled: Witnesses do not become zomemin ("scheming witnesses") until they falsify themselves . How so? A says: I testify that that man killed someone (on a certain day). B. says: How can you say that? That slain man (or that slayer) was with us on that day in this and this place? — These are not zomemin . But if they said: How can you testify thus? You were with us on that day in that place — These are zomemin , and they are killed for their testimony.