Revelation 13:16-18

16He also causes all—the small and the great, the rich and the poor, the free and the slave—to receive a mark on their right hand or upon their forehead.

17And so no one can buy or sell unless he has the mark—either the name of the beast or the number of his name.

18Here is wisdom: let the one who has understanding calculate the number of the beast, for it is a number of a man, and his number is 666.