Revelation 13:16-18

The second beast also forced all people, small and great, rich and poor, free and slave, to have a mark put on their right hand or on their forehead. No one could buy or sell without this mark. (This mark is the name of the beast or the number of its name.) Anyone who has understanding can find the meaning of the beast’s number. This requires wisdom. This number is the number of a man. It is 666.