Exodus 35:3

Kindle no fire. In early times the kindling of a fire required considerable labor. The comparatively warm climate of the Sinai region made artificial heating unnecessary, and a fire would have been kindled only for cooking purposes. Not being essential to health in such a climate, warm food was not to be prepared on Sabbath (see on ch. 16:23). This command is still strictly obeyed, even in cold climates, by the Karaite Jews, who permit neither fire nor light in their homes on the Sabbath day. However, many Jews view this charge as being only temporary in nature, and have both lights and fires, even in Palestine. But strictly orthodox Jews today cook no food on the Sabbath.