Ezra 2:13 Kabbalah

בְּנֵי֙ אֲדֹ֣נִיקָ֔ם שֵׁ֥שׁ מֵא֖וֹת שִׁשִּׁ֥ים וְשִׁשָּֽׁה׃ (ס)

The children of Adonikam, six hundred sixty and six.

Read kabbalistic (mystical) interpretation of Ezra 2:13: and classical mystical sources.