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.
Revelation 13:16-18 New International Version (NIV)
16 It also forced all people, great and small, rich and poor, free and slave, to receive a mark on their right hands or on their foreheads, 17 so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.
18 This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man.[a] That number is 666.