Revelation 13:18

Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.

Surrounding context

13:17 and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name.

13:18 Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.