Revelation 13:11-18 NKJV
Then I saw another beast coming up out of the earth, and he had two horns like a lamb and spoke like a dragon. And he exercises all the authority of the first beast in his presence, and causes the earth and those who dwell in it to worship the first beast, whose deadly wound was healed. He performs great signs, so that he even makes fire come down from heaven on the earth in the sight of men. And he deceives those who dwell on the earth by those signs which he was granted to do in the sight of the beast, telling those who dwell on the earth to make an image to the beast who was wounded by the sword and lived. He was granted power 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. 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