Revelation 13:16–18 (ASV)

¹⁶And he causeth all, the small and the great, and the rich and the poor, and the free and the bond, that there be given them a mark on their right hand, or upon their forehead; ¹⁷and that no man should be able to buy or to sell, save he that hath the mark, even the name of the beast or the number of his name. ¹⁸Here is wisdom. He that hath understanding, let him count the number of the beast; for it is the number of a man: and his number is Six hundred and sixty and six.