Rev 13:15-18

And it was given to him 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. (Rev 13:15)

And he causes all, small and great, rich and poor, free and bond, that they should receive marks on their right hand, or on their foreheads, (Rev 13:16)

and that no one may be able to buy or sell except he that has the mark, the name of the beast, or the number of his name. (Rev 13:17)

Here is wisdom. Let him that has understanding calculate the number of the beast, for it is the number of a man—his number is 666. (Rev 13:18)