Revelation 13:16-18

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