מדרש אויף במדבר 18:17

4) (Vayikra 27:33) ("He shall not discriminate between good and bad, and he shall not substitute for it; and if he did substitute for it, then it and its substitute shall be holy. It shall not be redeemed.") Because it is written (Devarim 12:11) "and all the choicest of your vows," I might think that he should "spy out" (his flock) and pick out the choicest (for ma'aser); it is, therefore, written "he shall not discriminate between good and bad." "and he shall not substitute for it": If he did, he receives forty lashes. "if substitute he shall substitute": to include his wife. "and if substitute he shall substitute": to include his heir. "then it and its substitute shall be holy. It shall not be redeemed": About a bechor it is written (Bamidbar 18:17) "you shall not redeem," (but) it is sold, when whole, alive. And when blemished, (it is sold) alive or slaughtered; but (it is) not (sold) slaughtered when whole. And about ma'aser it is written (here) "It shall not be redeemed." And it is not to be sold neither alive nor slaughtered; nor whole nor blemished.

5) It was said in the name of R. Yishmael: It is written (Bamidbar 18:17): But the bechor (the first-born) of an ox, or the bechor of a sheep, or the bechor of a goat, you shall not redeem. They are consecrated. Their blood shall you sprinkle upon the altar and their fat shall you smoke.": This teaches us that a bechor requires blood and fats upon the altar. Whence do we derive the same for ma'aser and pesach? From (Devarim 12:27): "And the blood of your sacrifices shall be spilled on the altar of your G d (first), and (then) the flesh shall you eat." This tells me only of their blood. Whence do I derive (the same for) their fat? From (Vayikra 3:16): "all the fat for the L–rd."

(Numb. 4:18:) “Do not cut off.” This text is related (to Ps. 33:18), “Behold, the eye of the Lord is on those who fear Him….” The text speaks along many lines of thought.¹²¹Shittim. For this use of the word, Buber, n. 209, cites Lev. R. 34:8. For the other interpretations, see above, Gen. 6:5. For what we need, however, it is speaking about the tribe of Levi.¹²²Numb. R. 5:1. And where is it shown? Where the tribe of Levi is called those who fear the Lord, as stated (in Mal. 2:5), “and I gave them (i.e., life and peace) as well as fear, and he feared Me.” (Ps. 33:19) “On those who wait for His steadfast love,” because they are always waiting for the name of the Holy One, blessed be He. (Ps. 33:19:) “To deliver their soul from death and to keep them alive in famine,” through the twenty-four gifts which the Holy One, blessed be He, has given them.¹²³THal. 2:7-9; BQ 110b (bar.); Hul. 133b (bar.); cf. Hal. 4:9. These are them: ten in the sanctuary, ten within the borders, and four in Jerusalem. The ten in the sanctuary: the sin offering (Lev. 6:17-23; Zev. 5:3), the guilt offering (Lev. 5:14-16, 20-26; 19:20-22; Zev. 5:5), the peace sacrifices and the community peace sacrifices (Lev. 23:19-20; Zev. 5:5), the sin offering of a fowl (Lev. 5:8), the guilt offering for a doubtful sin (Lev. 5:17-19; Zev. 5:5), the leper's log of oil (Lev. 14:12), the two loaves (Lev. 23:17), the shewbread (Exod. 25:30; Lev. 24:5-9), the remnant of the omer (Lev. 23:10-12; Men. 10:4), and the remainder of the meal offering (Lev. 2:3).
The ten within the borders: the terumah (Numb. 18:12), the terumah of the tithe (Numb. 18:25-29), the hallah (Numb. 15:18-21), the first of the shorn wool (Deut. 18:4), the shoulder, the cheeks, and the stomach (Deut. 18:3), the redemption of the [first-born] son (Numb. 18:15-16), [the redemption of] a firstling ass (Exod. 13:13), [the payment for] the robbery of a proselyte (Thal. 2:9; Bq 110b; Hul. 133b), things consecrated (Numb. 18:14; Bik. 3:12), and a field of possession (Lev. 27:16-21)
The four in Jerusalem: the firstlings [of animals] (Numb. 18:17-18), the first fruits (Exod. 23:19; Numb. 18:13; Hal. 4:9), the priest's share from the thank-offering ram and from the nazarite ram, the breast of the peace offerings, and the thigh (Exod. 29:27-28; Lev. 7:12-14; 31-34; 10:14-15; Numb. 6:13-20; 18:18), and skins of [burnt, sin, and guilt] offerings (Lev. 7:8; Zev. 12:3)
Behold, these are twenty-four gifts. Ergo (in Ps. 33:19), “and to keep them alive in famine. (Numb. 4:18) “Do not cut off [the tribe of the Kohathite families from the Levites].” The Holy One, blessed be He, foresaw that Korah was going to arise and disagree about the priesthood.¹²⁴Cf. Numb. R. 5:5. The Holy One, blessed be He, said. “I will not destroy the Levites because of Korah.” (Numb. 4:18:) “Do not cut off [the tribe of the Kohathite families from the Levites].” This text is related (to Is. 48:9), “For the sake of My name I will delay My anger, and for My praise I will hold back for you so as not to cut you off.” To what is the matter comparable?¹²⁵Numb. R. 5:6. To a king who had a son that was associated with bandits;¹²⁶Gk.: lestai. and when they were captured, his son was captured with them. The king said, “What shall I do? Shall I execute the robbers? Possibly my son is with them. Instead, for the sake of my son, I will exonerate them for now.” Similarly, the Levites carried the tabernacle. Thus it is stated (in Numb. 7:9), “But to the Children of Kohath he gave no [wagons], because they had the service of the holy.” When the Holy One, blessed be He, saw that Korah and his assembly were going to be opposed to Moses and Aaron, the Holy One, blessed be He, said, “What shall I do with these? To kill them in the desert is not possible.” Why? Because the Holy One, blessed be He, had taken half of His name and bestowed it upon them, the yh (of yhwh) in the Kohathite (hqhty in Numb. 4:18).¹²⁷Numb. R. 5:6, and Yalqut Shim‘oni, Is. 48:9, 326 (466) add that the letters from the divine name appear at the end and the beginning of HQHTY, and Numb. R. explains further that the Holy One added the definite article (H) to the name, Kohathite, for this very reason. It therefore says (in Is. 48:9), “For the sake of My name I will delay¹²⁸Literally: LENGTHEN. This verb may have suggested that the Holy One deliberately lengthened the name, Kohathite, with the addition of the article. My anger….”