Revelation 13:17-18

Young's Literal Translation

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.