Rev 13:16-18

16Also it causes all, both small and great, both rich and poor, both free and slave, to be marked on the right hand or the forehead,17so that no one can buy or sell who does not have the mark, that is, the name of the beast or the number of its name.18This calls for wisdom: let anyone with understanding calculate the number of the beast, for it is the number of a person. Its number is six hundred sixty-six.