The Sabbath Regulations, Exodus, Chapters 35 and 36, Sabbat Regulations

Moses assembled all the sons of israel and said to them, these are the things which the lord has commanded you to do. Six days shall work be done, but on the seventh day you shall have a holy sabbath of solemn rest to the lord whoever does any work on it shall be put to death. You shall kindle no fire in all your habitations. There is nothing, there is no greater human act that any one could do than the act of worship.