מדרש אויף תהילים 37:4

“It will be, because you heed these ordinances, and observe and perform them, that the Lord your God will maintain for you the covenant and the kindness regarding which He took an oath to your forefathers” (Deuteronomy 7:12).
Halakha: If a person of Israel has a candelabrum made of separate pieces, may one carry it on Shabbat? So taught the Sages: One who assembles the branches of a candelabrum on Shabbat incurs liability to bring a sin offering. For what is he liable? Rabbi Abbahu said in the name of Rabbi Yoḥanan: One who assembles the candelabrum on Shabbat is like one who builds on Shabbat, and one who builds on Shabbat is liable. Rabbi Yosei ben Rabbi Ḥanina said: When did Israel observe Shabbat properly? When it was first given to them in Alush. From where is this derived? As it is stated: “The people rested on the seventh day” (Exodus 16:30). [God said:] ‘Do you perhaps think that I gave you Shabbat to your detriment? I gave it only for your benefit.’ How so? Rabbi Ḥiyya ben Rabbi Abba said: [God said:] ‘You sanctify the Shabbat with food, drink, and clean garments, and bring pleasure to your soul, and I will give you reward.’ From where is this derived? “And you call Shabbat a delight…” (Isaiah 58:13). What is written thereafter? “Then you will delight in the Lord” (Isaiah 58:14). “[Delight in the Lord], and He will grant you the desires of your heart” (Psalms 37:4). Israel said to Him: ‘When will you give us the reward for the mitzvot that we perform?’ The Holy One blessed be He said to them: ‘The mitzvot that you perform, their fruits you consume now, but their reward I will give you at the end [be’ekev].’¹The word ekev literally means heel. Just as the heel is the end of the body, the word ekev in this context implies the end of time (Etz Yosef). From where is this derived? From what we read in our verse: “It will be, because [ekev] you heed.”

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."

Rabbi Levi said: If Israel observes Shabbat properly, even one day, the son of David²⁵The Messiah. will come. Why? Because it is equivalent to all the mitzvot. Likewise it says: “For He is our God, and we are the people of His pasture and the flock of His hand; today, would you only heed His voice” (Psalms 95:7). Rabbi Yoḥanan said: The Holy One blessed be He said to Israel: ‘Even though I set a limit for the end, and it will come whether they repent or do not repent, it will come only at its time. If they repent, even one day, I will bring it not at its appointed time. That is, “today, would you only heed His voice.” Just as we saw that for all the mitzvot, the son of David comes, for the observance of one day of Shabbat, the son of David comes, because Shabbat is equivalent to all mitzvot.
Rabbi Elazar bar Avina said: We find in the Torah, in the Prophets, and in the Writings that Shabbat is equivalent to all the mitzvot. In the Torah, from where is it derived? From the fact that when Moses forgot to tell them the mitzva of Shabbat, the Holy One blessed be He said to him: “Until when do you refuse to observe My commandments…” (Exodus 16:28). What is written thereafter? “See that the Lord has given you the Shabbat” (Exodus 16:28). In the prophets, from where is it derived? From that which is stated: “The house of Israel defied Me in the wilderness, they did not follow My statutes” (Ezekiel 20:13). What is written thereafter? “They desecrated My Shabbats” (Ezekiel 20:13). In the Writings, from where is it derived? From that which is stated: “You came down upon Mount Sinai and spoke to them [from heaven and gave them upright ordinances and laws of truth, good statutes and mitzvot]” (Nehemiah 9:13). What is written thereafter? “You informed them of Your holy Shabbat” (Nehemiah 9:14). The Holy One blessed be He said to Israel: ‘If you merit to observe Shabbat, I consider it for you as though you observed all the mitzvot of the Torah. If you desecrate it, I consider it for you as though you desecrated all the mitzvot.’ Likewise, it says: “Who keeps the Shabbat from its desecration and prevents his hand from performing any evil” (Isaiah 56:2).
When a person observes the Shabbat, he issues a decree and the Holy One blessed be He fulfills it, as it is stated: "If you restrain your walking because of the Sabbath” (Isaiah 58:13). What is written thereafter? “Then you will delight in the Lord” (Isaiah 58:14), just as it says: “Delight in the Lord, and He will grant you the desires of your heart.” (Psalms 37:4). Moreover, everything that you eat in this world is only from the profits, but the principal remains for the World to Come, as it is stated: “I will feed you the inheritance of Jacob your forefather, for the mouth of the Lord has spoken” (Isaiah 58:14).