Midrasch zu Bava Kamma 3:12

"Ye shall keep the Sabbath, for it is holy unto you" (Ex. 31:14). What is the keeping of the Sabbath? Neither to do any work thereon, nor to kindle fire thereon, neither to take forth nor to bring in beyond the Techum (limit) of the Sabbath even one foot, nor to fetch in his hand something which is not his food nor the food for his cattle. This is the keeping of the Sabbath, as it is said, "Wherefore the children of Israel shall keep the Sabbath" (Ex. 31:16).