Revelation 13:16-18 Modern English Version (MEV)
16 He causes all, both small and great, both rich and poor, both free and slave, to receive a mark on their right hand or on their forehead, 17 so that no one may buy or sell, except he who has the mark or the name of the beast or the number of his name.
18 Here is a call for wisdom: Let him who has understanding calculate the number of the beast. It is the number of a man. His number is six hundred and sixty-six.