Revelation 13:18 YLT

¹⁶  And it maketh all, the small, and the great, and the rich, and the poor, and the freemen, and the servants, that it may give to them a mark upon their right hand or upon their foreheads,

¹⁷  and that no one may be able to buy, or to sell, except he who is having the mark, or the name of the beast, or the number of his name.

¹⁸  Here is the wisdom! He who is having the understanding, let him count the number of the beast, for the number of a man it is, and its number 'is' 666.