The first mention of Sabbath is in Exodus 16.
Ex 16:22-30:
22 On the sixth day, they picked up twice as much bread, two omer measures for each person. So all the chieftains of the assembly came and reported it to Moses. 23 At that he said to them: “It is what Jehovah has said. Tomorrow there will be a complete rest, a holy sabbath to Jehovah. Bake what you need to bake, and boil what you need to boil; then save whatever is left over and keep it until the morning.” 24 So they saved it until the morning, just as Moses had commanded, and it did not stink nor were there maggots in it. 25 Then Moses said: “Eat it today, because today is a sabbath to Jehovah. Today you will not find it on the ground. 26 You will pick it up for six days, but on the seventh day, the Sabbath, there will be none.” 27 However, some of the people did go out to pick it up on the seventh day, but they found nothing.
28 So Jehovah said to Moses: “How long will you refuse to keep my commandments and my laws? 29 Take notice of the fact that Jehovah has given you the Sabbath. That is why he is giving you the bread for two days on the sixth day. Everyone must stay where he is; nobody is to leave his locality on the seventh day.” 30 So the people observed the Sabbath on the seventh day.
Ex 20:8-11:
“Remember the Sabbath day to keep it sacred. 9 You are to labor and do all your work for six days, 10 but the seventh day is a sabbath to Jehovah your God. You must not do any work, neither you nor your son nor your daughter nor your slave man nor your slave girl nor your domestic animal nor your foreign resident who is inside your settlements. 11 For in six days Jehovah made the heavens and the earth, the sea, and all that is in them, and he began to rest on the seventh day. That is why Jehovah blessed the Sabbath day and made it sacred.
Ex 31:14-17:
You must keep the Sabbath, for it is something holy to you. Whoever profanes it must be put to death. If anyone does any work on it, then that person must be cut off from among his people. 15 Six days work may be done, but on the seventh day is a sabbath of complete rest. It is something holy to Jehovah. Anyone doing work on the Sabbath day must be put to death. 16 The Israelites must keep the Sabbath; they must observe the Sabbath during all their generations. It is a lasting covenant. 17 It is an enduring sign between me and the people of Israel, for in six days Jehovah made the heavens and the earth and on the seventh day he rested and refreshed himself.’”
Ex 35:1-3:
Moses later gathered the entire assembly of the Israelites together and said to them: “These are the things that Jehovah has commanded to be done: 2 Work may be done for six days, but the seventh day will become something holy to you, a sabbath of complete rest to Jehovah. Anybody doing work on it will be put to death. 3 You must not light a fire in any of your dwelling places on the Sabbath day.
You get the idea.