Revelation 13:16-18

Also he compels all, the small and the great, and the rich and the poor, and the free men and the slaves, to be given a mark on their right hand or on their forehead [signifying allegiance to the beast], and that no one will be able to buy or sell, except the one who has the mark, either the name of the beast or the number of his name. Here is wisdom. Let the person who has enough insight calculate the number of the beast, for it is the [imperfect] number of a man; and his number is six hundred and sixty-six.