Midrash su Levitico 27:26

1) (Vayikra 27:26) ("But a first-born ["bechor"] which is a firstling ["asher yevukar"] to the L-rd among the beasts, a man shall not consecrate it [as a different offering]. Whether ox or lamb, it is the L-rd's.") If it were just written "a first-born … shall not consecrate," I might understand it as: a human first-born shall not make consecrations; it is, therefore, written ("a man shall not consecrate) it" (a first-born beast) — "it" you shall not consecrate, but a human first-born may make consecrations. But I still might understand it as: he (a human first-born) shall not consecrate it, but others may; it is, therefore, written "among the beasts" — It is a first-born beast that is being referred to (and not a first-born human being).

R. Yishmael says: One verse (ours) says: "You shall consecrate," and another, (Vayikra 27:26) "you shall not consecrate"! You can consecrate it (a bechor) for its valuation to Temple maintenance; but you cannot consecrate it (as a different offering) to the altar. This tells me only of a bechor. Whence do I derive (the same for) all the offerings? From "in your cattle and in your sheep … you shall consecrate." Or, something which you learn as obtaining with one thing, you may learn as obtaining with all (similar) things, viz.: Just as a bechor is distinct in being a lower-order offering, and it is eaten for two days, and applies to cattle and sheep — so, (this obtains with) all that are thus characterized (i.e., with lower-order offerings). And whence do I derive (the same [i.e., that it is forbidden to shear or work with them]) for (other offerings which also obtain with cattle and sheep, such as) holy of holies, and individual and communal lower-order offerings? From "in your cattle and in your sheep … you shall consecrate." I might think that this (that they may not be shorn or worked with) applies also to consecrations for Temple maintenance; it is, therefore, written (Devarim 15:19) "the bechor." Was bechor not included in all offerings? Why was it thus singled out? To teach: Just as bechor is distinct in being an altar offering, etc. — to exclude Temple maintenance consecrations, which are not altar offerings.