Joseph Gikatilla, in Heikhal ha-Shem (Venice, 1601), 40a

Jewish Mysticism
Sexual intercourse is the secret of holiness (sod ha-qedushah) and the multiplication of the [divine] image, and its opposite is the diminution of the image. Therefore, sexual intercourse on Friday evenings is the time of the conjugal obligation of the scholar, “who yields his fruit in season” (Ps. 1-3), in the secret of zakhor and shamor, and the secret of “you must keep My Sabbaths” (Exod. 31-13). Since the extra soul (neshamah yeterah) is added to a person on Sabbath eves, then is the time for the sanctification of the union (qedushat ha-hibbur) and worthy children [shall be born] on behalf of the “eunuchs who keep my Sabbaths” (Isa. 56-4).

Translated by Elliot Wolfson in, “Coronation of the Sabbath Bride- Kabbalistic Myth and the Ritual of Androgynisation,” Journal of Jewish Thought and Philosophy 6 (1997)- 301-344.