זוהר בראשית — דף קנט ע”א

The Zohar explains that the Holy One, blessed be He, made the lower world like the upper one: just as there are watches on earth, so in the firmament beings sing continually to their Master. These are the “maidens of song,” whose number cannot be counted. They stand in three orders for each of the four sides of the world, each order containing three more, and they move by means of marked letters. Two upper letters, mercy and judgment, belong to the masculine; a third rises from below and joins them, as the feminine, corresponding to the letters Yod-He-Vav. One who knows them and takes care with them is beloved above and below. Rabbi Shimon adds that they are included in the perfect union of the upper and lower waters. The text then describes the orders of the south and north, twenty-seven in all.