Revelation 4:8; Revelation 13:18

8 And the four living creatures, each one of them having six wings, are full of eyes around and within; and day and night they do not cease to say, "HOLY, HOLY, HOLY is THE LORD GOD, THE ALMIGHTY, WHO WAS AND WHO IS AND WHO IS TO COME."

18 Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is six hundred and sixty-six .