Revelation 13:18 WEB

¹⁶  He causes all, the small and the great, the rich and the poor, and the free and the slave, to be given marks on their right hands, or on their foreheads;

¹⁷  and that no one would be able to buy or to sell, unless he has that mark, the name of the beast or the number of his name.

¹⁸  Here is wisdom. He who has understanding, let him calculate the number of the beast, for it is the number of a man. His number is six hundred sixty-six.