Revelation 13:16-18

And she will cause everyone, small and great, wealthy and poor, free and servant, to have a character on their right hand or on their foreheads, so that no one may buy or sell, unless he has the character, or the name of the beast, or the number of his name. Here is wisdom. Whoever has intelligence, let him determine the number of the beast. For it is the number of a man, and his number is six hundred and sixty-six.