Revelation 13:16-18 NKJV
He causes all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads, and 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.
Here 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.
NKJV: New King James Version