BT Nid 66a(51) :היכ׳ דאחמור אחמור היכ׳ דלא אחמור לא אחמור‏ : where they applied a restriction (not required by the law) they have done so [i.e. it applies]. Where they did not apply a restriction, they have not done so [i.e. it does not apply]
See the entire entry for ḥmr vb. to become hard