Revelation 13:16-20

And he causes all, the small and the great, the rich and the poor, and the free and the slaves, to be given a mark on their right hands or on their foreheads,

and he decrees that no one will be able to buy or to sell, except the one who has the mark, either the name of the beast or the number of his name.

Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is six hundred and sixty-six.