Midrash su Salmi 37:4

Rabbi Jochanan in the name of R. Jose said: "To him who observes the Sabbath, with enjoyments, will be given boundless inheritance, as it is written (Is. 58, 14.) Then Shalt thou find delight in the Lord, etc., And I will cause thee to enjoy the inheritance of Jacob, thy father. Not such inheritance as was promised to Abraham, (Gen. 13. 14) Arise and walk through the land to its length and breadth, and not as it was promised to Isaac (Ib. 26, 3.) I will give thee all that this land contains, but as it was promised to Jacob (Ib. 28, 14.) And thou shall spread abroad, to the West, and to the East, to the North, and to the South." R. Nachman b. Isaac said: "He will be saved from the subjugation of exile; for it is written (Is. 58, 14.) And I will cause thee to tread upon the high places of the earth; it is also written there (Deu. 33, 39.) And thou shalt tread upon their high places." R. Juda said in the name of Rab: "To him who observes the Sabbath, with enjoyment, will be granted his heart's desires, for it is said (Ps. 37, 4.) And delight thyself in the Lord, and He will give thee the wishes of thy heart. The word delight is not understood in its real meaning. Since it says (Is. 58, 13.) And if thou call the Sabbath delight; then we knew that it means delight of the Sabbath. R. Chiya b. Abba in the name of R. Jochanan said: "He who observes the Sabbath properly, according to its commandment, even if he worship idols, as did the generation of Enosh, will be pardoned, for it is said (Is. 56, 2) Happy is the man that ever doth thus, etc., by not violating it. Do not read it, M'chalelo (violating it), but read it Machul-lo (pardoned) him)." R. Juda, in the name of Rab, said: "If Israel had strictly observed the first Sabbath, no nation or race would have been able to dominate over them, for it is said (Ex. 16, 27.) And it came to pass on the seventh day, that there went out some of the people to gather (the Manna) but did not find them. Immediately following is written: Then came Amalek." R. Jochanan in the name of R. Simon b. Jochai. said: "If Israel would observe two Sabbaths, only, according to the strict requirements of the law, they would at once be redeemed, for it is written (Is. 56, 4.) Thus saith the Lord unto the eunuchs that keep my Sabbaths, and immediately following is written, Even them will I bring to my holy mountain."

According to Rabbi Levi: If all of Israel were to keep Shabbat properly for even a single day, the Messiah ben David would come. Why? because it is equivalent to all of the commandments, as it is said (Ps. 95:7) "for He is our God, and we are the people He tends, the flock in His care. O, if you would but heed His charge this day:"