Midrash sa Malakias 3:3: Ein Yaakov (Glick Edition), Vayikra Rabbah, Midrash Tehillim & Bamidbar Rabbah

Rabba b. Huna said: "There is one great preference between an Israelite and a Proselyte. Concerning the Israelite the passage reads (Ez. 37, 27) And I will be their God, and they shall be My people, but concerning a Proselyte it is written (Jer. 30, 21) For who is this that has pledged his heart to approach unto Me?" R.Chelbo said:"Proselytes are as bad to Israel as a sore on the skin; for it is written (Is. 14, 1) And the stranger shall be joined unto them, and they shall attach (V'niss'pechu) themselves to the house of Jacob. It is written here [in the Text] V'niss'pchu, and it is written [concerning leprosy] the same expression (Lev. 14, 56) And for a swelling and for a rising (Sapachath)." R. Chama b. Chanina said: "At the time when the Holy One, praised be He, (Fol. 71) will purify the tribes He will commence with the tribe of Levi first, as it is said (Mal. 3, 3) And He will sit as a melter and purifier of silver; and He will purify the sons of Levi, and refine them as gold and silver, that they may offer unto the Lord an offering in righteousness." R. Joshua b. Levi said: "Impure families who joined Israel through the influence of their wealth, will be purified, as it is said (Mal. 3, 3) And He will sit as a melter and purifier of silver. What does the passage (Ib.) That they may offer unto the Lord an offering in righteousness, mean? Said R. Isaac: "Righteousness has the Lord done with Israel that every family which was mixed up beyond recognition is considered pure."

Another matter, “if he sacrifices it as a thanks offering” – that is what the verse said: “A fool proclaims his guilt” (Proverbs 14:9). Rabbi Yudan said: The fool articulates his guilt with his mouth and says: ‘Is it not a sin offering that I am liable to bring? Is it not a guilt offering that I am liable to bring?’¹²He minimizes his sin by saying that he is merely required to bring an offering and all is fine. Rabbi Yudan said in the name of Rabbi Levi: These are people who conduct themselves permissively with maidservants in this world¹³They engage in sexual relations with maidservants, reasoning that at most they will bring the required offering and be forgiven. and the Holy One blessed be He hangs them by the crown of their heads in the future. That is what is written: “Indeed, God will crush the heads of His enemies, hairy skulls of those walking in their guilt” (Psalms 68:22). All the people will say: Let that person go in his sin, let that person go in his sin.
Another matter, “a fool proclaims his guilt,” this is one who brings his offering but does not gain atonement. What shall he do? He shall go to the tribe of Levi, in whose regard it is written: “He will sit refining and purifying silver and he will purify the sons of Levi…[and they will be for the Lord, presenters of an offering in righteousness]” (Malachi 3:3). “But among the upright there is good will” (Proverbs 14:9), this is one who brings an offering not because of a sin; “if he sacrifices it as a thanks offering.”

The sayings of the Lord are pure sayings. Rabbi Joshua ben Levi said: The Scripture established two or three expressions and did not utter anything improper. As it is said, "Of the clean animal, and of the animal that is not clean" (Genesis 7:8). Rabbi Tanchum said: The Holy One, blessed be He, spoke two expressions to you and they are pure. To whom did He give the tribe of Levi? To the one of whom it is written, "And he shall sit as a refiner and purifier of silver" (Malachi 3:3). And which is the section that deals with the dead? The one that begins, "Speak unto the priests, the sons of Aaron, and say unto them" (Leviticus 21:1). Rabbi Yannai said: The words of the Torah were given in concise language, but for every matter that the Holy One, blessed be He, spoke to Moses, He would speak from two aspects: one clean and one unclean. He said before Him, "Master of the Universe, until when will we stand for the purification of the law?" He replied, "After the majority, incline." The majority of the judges who rule that the unclean is unclean, and the clean is clean. Rabbi Abbahu said in the name of Rabbi Jonathan: Rabbi Akiba had an advanced disciple who would render the unclean unclean with one expression, and the clean clean with one expression. So too did Rabbi Meir have a disciple named Sumchus, who would render the unclean unclean with one expression, and would then render it clean with one expression, as it is said, "He is pressed, he is seven times refined" (Psalms 12:7). Rabbi Joshua said in the name of Rabbi Levi: In the days of David, even infants knew how to expound the Torah in two aspects, as it is written in Psalm 7. (source: Sotah 42a, translation adapted from sefaria.org)