Tosefta su Deuteronomio 18:4

A Kohen who takes upon himself all the ritual obligations of the priesthood, except for one thing, we do not accept him. A Levite who takes upon himself all the Levitical obligations except for one thing, we do not accept him, as it is written (Lev. 7:33), "he who offers the blood of the peace-offering, and the fat [of the peace-offering] from among the sons of Aaron shall get the right thigh as his portion." [With reference to Kohanim (following Steinsaltz to Hul. 132b:15 to Hul. 133a:2)], I have only derived [that a Kohen does not share in the priestly gifts] only [if he does not believe in the validity of] sprinkling blood and burning of fats [on the altar]. How do we know [that they are also obligated in the rites of] the pouring [of oil onto the minchah-offering], the mixing [the ingredients together], the waving and bringing [the meal offering to the corner of the altar], the removing the handful and burning [on the altar], cutting [the neck of bird-sacrifices], receiving and sprinkling [the blood], the Sotah ceremony, the breaking the red heifer's [neck], and purifying the metzora [= leper], and the lifting the hands [to bless the people] inside and outside [the Temple]? As the Torah teaches (Lev. 7:33) "[among the] sons of Aaron," that is [to say] "all the rites" that are incumbent upon the sons of Aaron. Said Rabbi Shimon, you might think [lit. "it can be"] we don't prohibit them except from partaking in the priestly gifts within the Temple; how do we know [we also prohibit them partaking in priestly gifts given] in the countryside (see Y. Chal. IV.4.17)? As the Torah teaches (Deut. 18:4), "You shall also give him the first fruits of your new grain [and wine and oil, and the first shearing of your sheep]." Because [as it says, Deut. 18:5], "For the LORD your God has chosen him [etc.]"; thus, everyone who takes upon himself the service, he receives the gifts. All those who do not take upon themselves the service, he does not receive the gifts. During the time that the Kohanim do the will of [their Father in heaven, following the GR"A], what it says about them (Lev. 6:10), "I have given it as their portion from My offerings by fire." What is theirs, they take, and they do not take what is mine. And during the time that they do not do His will, what is said about them (Mal. 1:10), "If only you would lock My doors[, and not kindle fire on My altar to no purpose! I take no pleasure in you—said the LORD of Hosts—and I will accept no offering from you.]"

[The laws pertaining to] "first of the fleece" (Deut. 18:4) apply to tereifah [sheep], and do not apply to dead [animals], and do apply to kilayim (the mixed-breed offspring of a sheep and another animal) as well as to a k'vi. Rabbi Eliezer says, as to kilayim, it applies to [the offspring of] a ewe and a goat, but it does not apply to a k'vi. A Kohen and an idolater that [jointly] gave their animal to an Israelite, [the animal] is exempt from "first of the fleece," and this is a stringency pertaining to the cheeks and the maw (see Deut. 18:3) that does not apply to "first of the fleece" [as the Israelite would remain liable to donate the cheeks and the maw to the Kohen following slaughter in this scenario (see Minchat Yitzchak)]. All consecrated [animals] that had a permanent blemish before they were consecrated and [then] were redeemed are liable in "first of the fleece," but if they were consecrated before their blemish [developed], or [if] their temporary blemish preceded their consecration and afterwards they developed a [permanent] blemish, and they were redeemed, they are exempt from "first of the fleece." One who shears a goat is exempt from "first of the fleece." One who washes his ewes [and wool comes out during the washing] is liable in "first of the fleece." One who purchases [a sheep] from a homeowner is liable, and Rabbi Ilai exempts him. Partners [who share the profits of a sheep's wool] are [jointly] liable. Rabbi Akiva exempts them. [Regarding the purchase of a single sheep,] Rabbi Ilai says, there is no "first [of the fleece]" unless there is a "last [of the fleece]" after it. [How many sheep render one liable?] [The midrashic exposition of the phrase "the fleece of your sheep" yields a minimum of] four [according to the school of Rabbi Yishmael son of Rabbi Yosei (see Hul. 137a:20)], [while the midrashic exposition of the phrase] "you shall give him" [yields a minimum of] five [according to Beit Hillel (see Hul. 137a:17-19, and see Minchat Yitzchak here)]. One who possesses the wool of five ewes [worth] a maneh (one hundred dinars) and a half each, which weigh five selahs in Judean measurements, which are ten selahs in Galilean measurements -- he is liable in "first of the fleece," the words of Rabbi Dosa. And the Sages say, the fleece of five sheep, regardless of the value [of each fleece], weighing five selahs in Judean measurements, which are ten selahs in Galilean measurements, [when weighed after being] laundered and not [when they are] dirty. Not that one must launder the wool [himself], but must give the Kohen a sufficient quantity so that after [the Kohen] launders it, he remains in possession of [wool weighing at least] five selahs in Judean measurements, which are ten selahs in Galilean measurements, as it says, "Give (titen) it to him," so that he will have a quantity [commensurate with] a gift (matanah). One who shears two sheep and leaves off, then shears [again] and leaves off, even though in the future [the wool] will add up [to the fleece of five sheep], we do not join [the shearings] together. If he sheared five [sheep] and leaves off, even if [when he resumes shearing he shears only] two and then another two, behold, these do join together. One who separates the fleece of his sheep, and lost [the separated portion], he is liable for its monetary value. "First of the fleece" -- the commandment applies to the beginning [of the shearing season]. [But bedi'eved,] if he gave [the first of his fleece], whether at the beginning, the middle, or the end, he has fulfilled [the obligation]. What is the required measure? One in sixty, just like terumah. One who purchases the fleece of his fellow's sheep and does not separate the first of the fleece from it, the buyer is exempt. This is a stringency in the shankbone, the cheeks, and the maw [where the buyer remains liable] that does not exist with first of the fleece. [With respect to the commandment to send away the mother bird (Deut 22:6-7, discussed below)], geese, chickens, and Herodian pigeons that descended upon or nested in an orchard are liable [in the commandment of] "sending away."