Revelation 13:14-18

14And he deceives those who dwell on the earth by those signs which he was granted to do in the sight of the beast, telling those who dwell on the earth to make an image to the beast who was wounded by the sword and lived.

15He was granted power to give breath to the image of the beast, that the image of the beast should both speak and cause as many as would not worship the image of the beast to be killed.

16He causes all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads,

17and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name.

18Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.