Revelation 13:16-18

He also caused everyone (small and great, rich and poor, free and slave) to obtain a mark on their right hand or on their forehead. Thus no one was allowed to buy or sell things unless he bore the mark of the beast – that is, his name or his number. This calls for wisdom: Let the one who has insight calculate the beast’s number, for it is man’s number, and his number is 666.