Revelation 13:18

14He deceives my own people who dwell on the earth because of the signs he was granted to do in front of the beast, saying to those who dwell on the earth that they should make an image to the beast who had the sword wound and lived.

15It 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 wouldn’t worship the image of the beast to be killed.

16He causes all, the small and the great, the rich and the poor, and the free and the slave, to be given marks on their right hands or on their foreheads;

17and that no one would be able to buy or to sell unless he has that mark, which is the name of the beast or the number of his name.

18Here is wisdom. He who has understanding, let him calculate the number of the beast, for it is the number of a man. His number is six hundred sixty-six.