Apocalypse 13:16-18

And he shall make all, both little and great, rich and poor, freemen and bondmen, to have a character in their right hand, or on their foreheads. And that no man might buy or sell, but he that hath the character, or the name of the beast, or the number of his name. Here is wisdom. He that hath understanding, let him count the number of the beast. For it is the number of a man: and the number of him is six hundred sixty-six.