Revelation 13:16-18

¹⁶ It puts under compulsion all people – the small and the great, the rich and the poor, the free and the slaves – that these should be marked on their right hand or on their forehead, ¹⁷  and that nobody can buy or sell except a person having the mark, the name of the wild beast or the number of its name. ¹⁸ This is where it calls for wisdom: Let the one who has insight calculate the number of the wild beast, for it is a man’s number, and its number is 666.