Deuteronomy 18:3

And this shall be the portion belonging to the priest from the people, from them who offer sacrifices, whether bullock or lamb they shall give to the priest the right shoulder, the lower jaw, the cheeks, and the maw;

AND THIS SHALL BE THE PRIESTS’ DUE FROM THE PEOPLE. This is a new commandment not mentioned in [the preceding books of] the Torah, for, in the wilderness where they did not slaughter oxen or sheep except as peace-offerings, He did not mention it because it does not apply to consecrated animals. Now, as they were about to enter the Land, he declared it to them for the first time. This is the sense of the expression [And this shall be the priests’ due from the people] from them that slaughter the animal. That is to say, when they will slaughter an animal as I have permitted you, [as it is said], then thou mayest slaughter of thy herd and of thy flock as I have commanded thee, and thou mayest eat in thy gates [you shall give the priest his due the shoulder, and the two cheeks, and the maw]. He did not allocate this [gift of the shoulder, cheeks, and maw] to Aaron in the section, And the Eternal said unto Aaron [wherein the other priestly gifts are enumerated], although there He assigned the heave-offerings and the first-fruits [despite the fact that they, like the gifts in the verse before us] apply only in the Land. The reason [for mentioning them there] is that there He stated all things that He gave the priests in sacred matters, for even things “devoted” are holy., Verse 14. See Leviticus 27:28. But the shoulder, and the two cheeks, and the maw, and the first of the fleece [mentioned here] are completely unconsecrated, since there are no special laws regarding them aside from the commandment of giving them to the priest. Similarly He did not mention there [in the chapter of the priestly gifts] the law of property wrongfully taken from a proselyte [which reverts to the priests if the proselyte died without leaving an heir, and the reason this law is not mentioned in the chapter of the priestly gifts is, as explained concerning the gift in the verse before us, that the reverted property of the proselyte is not sacred].
But according to the interpretation of our Rabbis, all [of the twenty-four priestly gifts] are alluded to there [in the section, And the Eternal said unto Aaron]. Thus they have said: 117. “The first part of them — this refers to the first of the fleece. Which they give — this refers to the shoulder, and the two cheeks, and the maw. Unto the Eternal — this refers to the dough-offering.” (ibid., 15:19). If so, this also is an explanatory …

מאת זובחי הזבח, “from those that offer a sacrifice.” The Torah refers to the priests slaughtering the animal in question. The word זבח is not to be understood literally as the sacrifice, [which is either owned by an individual or is a communal offering. Ed.] The reason that must be so is that the portions of the animal assigned for the priest by the Torah, i.e. a foreleg, the stomach and the jaws, are not animals that have been sanctified as offerings, but are animals that were slaughtered privately, after offering sacrifices on private altars was no longer permitted, and they could be slaughtered only in the town where either the Tabernacle or the Temple stood.

זובחי, the plural mode is used here in order to include animals owned by partners.