מדרש על מכות 9:5

We 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.