Musar על שבת 139:3

When G–d began the work of Creation, He said: "Let there be light," the foundation of fire. There is both visible and hidden fire. This is the mystical dimension of the Torah's commandment in Exodus 35,3: לא תבערו אש בכל מושבותיכם ביום השבת. "Do not light a fire in any of your dwellings on the Sabbath day." The Torah singled out this prohibition of work (whereas none of the other 38 basic prohibitions of work is named).