Revelation 13:16-18 New American Standard Bible (NASB)
16 And he causes all, the small and the great, and the rich and the poor, and the free men and the slaves, [a]to be given a mark on their right hand or on their forehead, 17 and he provides that no one will be able to buy or to sell, except the one who has the mark, either the name of the beast or the number of his name. 18 Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is [b]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.