זוהר שמות — דף רח ע”א

After Shabbat ends, a person must make Havdalah to distinguish between the holy and the mundane, as permission has been given to the appointed beings to rule. In Havdalah, a person demonstrates the unity of a holy place within supernal holiness. The secret of Havdalah is in the candle, whose lights beneath this holiness unite into one. The cup used in Havdalah is a cup of blessing, because the supernal blessing resides in it. On Shabbat one must not engage in any labor, and must preserve the holiness of the place in all of one's actions.