The Book of Revelation: Chapter 13 NKJV
The Mark
Verse 16: "And he causers all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads.
Verse 17: "And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.

The number is 666.