Revelation 13:16–18 (DARBY)

16  And it causes all, the small and the great, and the rich and the poor, and the free and the bondmen, that they should give them a mark upon their right hand or upon their forehead; 17  and that no one should be able to buy or sell save he that had the mark, the name of the beast, or the number of its name. 18  Here is wisdom. He that has understanding let him count the number of the beast: for it is a man’s number; and its number is six hundred and sixty-six.