Midrash su Malachia 3:4

... Another explanation. “When the Lord, your God, expands your boundary…” (Deuteronomy 12:20) The Rabbis say: this is speaking of Jerusalem. Who is able to see the calm of Jerusalem when the Holy One expands it? R’ Shimon bar Nachman says: to what is this to be compared? To a country, etc. “And then the offerings of Judah and Jerusalem shall be pleasant to the Lord, as in the days of old and former years.” (Malachi 3:4) “Lo, I will send you Elijah the prophet before the coming of the great and awesome day of the Lord, that he may turn the heart of the fathers back through the children, and the heart of the children back through their fathers-lest I come and smite the earth with utter destruction.” (Malachi 3:23-24) Behold I send My angel and he will clear the way before Me. And suddenly the Master whom you desire will come into His palace and the angel of the covenant whom you desire, behold he is coming says the Lord of Hosts. “Therefore, so said the Lord: 'I have returned to Jerusalem with mercy; My house shall be built there,' says the Lord of Hosts. 'And a plumb line shall be stretched out over Jerusalem.' Further, proclaim, saying: so said the Lord of Hosts, 'My cities shall yet spread out with prosperity, and the Lord shall yet console Zion and shall yet choose Jerusalem.'” (Zechariah 1:16-17) “Be exceedingly happy, O daughter of Zion; Shout, O daughter of Jerusalem. Behold! Your king shall come to you. He is just and victorious; humble, and riding a donkey and a foal, the offspring of she-donkeys.” (Zechariah 9:9)

“Return us to You, Lord, and we will return; renew our days as of old” (Lamentations 5:21).
“Return us to You, Lord, and we will return.” The congregation of Israel said before the Holy One blessed be He: ‘Master of the universe, it is incumbent upon You to return us.’ He said to them: ‘It is incumbent upon you, as it is stated: “Return to Me and I will return to you, said the Lord” (Malachi 3:7).’ It said before Him: ‘Master of the universe, it is incumbent upon you, as it is stated: “Return us, God of our salvation” (Psalms 85:5).’ That is why it is stated: “Return us to You, Lord, and we will return.”
“Renew our days as of old [kekedem].” Like Adam the first man, just as it says: “He banished the man; He stationed…east [mikedem] of the Garden of Eden” (Genesis 3:24).²⁸Adam repented after being banished from Eden, and his repentance was accepted (Rabbi David Luria). Alternatively, “renew our days as of old.” Just as it says: “The offering of Judah and Jerusalem will be pleasant to the Lord, as in the days of old and as in former years” (Malachi 3:4). “As in the days of old,” this is Moses, as it is written: “He remembered the days of old, Moses, His people” (Isaiah 63:11). “And as in former years,” like the years of Solomon. Rabbi [Yehuda HaNasi] says: “As in the days [kimei] of old,” as in the days of Noah, as it is stated: “For, like the waters of [ki mei] Noah, this is for Me” (Isaiah 54:9). “And as in former years,” like the years of Abel, when there was not yet idolatry in the world.

Rabbi Yehuda interpreted the verse regarding Torah scholars: “Curls, black as a raven” – these are Torah scholars, as even though they appear ugly and black in this world, in the future, “their appearance is like torches, they dash like lightning” (Nahum 2:5).
Rabbi Shmuel bar Yitzḥak interpreted the verse regarding the portions of the Torah: “Curls, black as a raven” – these are the texts of the Torah that appear too ugly and black to state them in public, and the Holy One blessed be He said: They are pleasant for Me, just as it says: “The offering of Judah [and Jerusalem] will be pleasant” (Malachi 3:4). Know that it is so, as the portion of the zav and the zava were not stated together; rather, this one in and of itself and that one in and of itself, as it is stated: “Speak to the children of Israel, and say to them: Any man, when he has a discharge from his flesh” (Leviticus 15:2), [and separately,] “And a woman, if her bloody discharge shall flow” (Leviticus 15:25).

4 (Numb. 15:3) “And you shall make an offering to the Lord, a burnt offering, or a sacrifice”: The Holy One said to them, “Whoever offers Me a sacrifice in this world, does not offer it in vain.¹⁰See Sifra to Lev. 9:24 (99: Parashat Shemini Mekhilta deMiluim); Lam. R. 5:1 (1); Lev. R. 7:96; also Deut. R. 4:11. Rather [if] he offers it, it is pleasing to Me. Also in the world to come, he shall have the right to make an offering. Then I will accept it, and it shall be pleasing to Me.” It is so stated (in Mal. 3:4), “Then shall the offering of Judah and Jerusalem be pleasing to the Lord, as in the days of old and as in the former years.”

(Numb. 15:3:) AND YOU SHALL MAKE AN OFFERING TO THE LORD, A BURNT OFFERING, OR A SACRIFICE…. The Holy One said to them: Whoever offers me a sacrifice in this world, does not offer it in vain.⁵³Numb. R. 17:4; see Sifra to Lev. 9:24 (99: Parashat Shemini Mekhilta deMiluim); Lam. R. 5:1 (1); Lev. R. 7:96; also Deut. R. 4:11. Rather [if] he offers it, it is pleasing to me. Also in the world to come, he shall have the right to make an offering. Then I will accept it, and it shall be pleasing to me. It is so stated (in Mal. 3:4): THEN SHALL THE OFFERING OF JUDAH AND JERUSALEM BE PLEASING TO THE LORD, AS IN THE DAYS OF OLD AND AS IN THE FORMER YEARS.

31) (Vayikra 9:24): "And a fire came forth from before the L–rd, and it consumed upon the altar the burnt-offering and the fats. And all the people saw and they exalted (the L–rd), and they fell upon their faces.": When they saw a new fire which descended upon the altar and lapped up the burnt-offering and the fats, they fell upon their faces and praised the L–rd. And in the days of Solomon, too, they fell upon their faces, as it is written (II Chronicles 7:3): "And all the children of Israel saw when the fire came down and the glory of the L–rd was upon the house. And they knelt on the floor with their faces to the ground, and they prostrated themselves in thanks to the L–rd — (saying:) 'For He is good; for His lovingkindness endures forever.'" And of that time it is written (Malachi 3:40: "Then the offerings of Judah and of Jerusalem shall be as pleasant to the L–rd as in days of old and as in former years.": "as in days of old" — the days of Solomon; "and as in former years" — the years of Moses. Rebbi says: "as in days of old" — the days of Noach (viz. Bereshith 8:21); "and as in former years" — the years of Hevel (viz. Bereshith 4:4), when idolatry had not yet appeared in the world.