Ma‘arekhet ha-’Elohut, 120a.

Jewish Mysticism
Since intercourse is considered profane in relation to Adam after he had sinned, and it is performed through the sign of the holy covenant, it is appropriate not to engage in sexual intercourse except for the nights of Sabbath in order not to make from that which is holy something profane. On the eve of Sabbath he is in his potency and intercourse is then holy, for the days of the week, wherein the shells surround them below, are completed and finished on the day of Sabbath. This is the secret of [the tradition that] the time for scholars to fulfill their marital obligation is on the eve of Sabbath.

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.