Tossefta sur Les Nombres 8:8

A bull of twenty-four months and one day, behold, that is a "wholly sound bull," and Rabbi Eliezer says, we give him thirty more days after twenty-four months, for wherever it says "a young bull of the herd" (see Lev. 4:3), it means a two year-old. If it just says "bull," that is a three year-old, up until a five year-old. Rabbi Yosei HaGalili says, bulls must be two (sh'tayim) years old, as it is said (Num. 8:8), "And a bull sheni, a young of the herd, take for a sin-offering." They said to him, it doesn't mean sheni as in "two"; rather, sheni as in "second" [in comparison] to the "first." Just as the first [bull] is not eaten, so too the second bull is not eaten. Said Rabbi Shimon, why is a sin-offering like an intercessor (p'raklit)? For it enters [the palace] to appease [the king]. [After] the intercessor has appeased [the king], the present (i.e., the burnt-offering) follows.

Rebbi says, what is the verse coming to teach (Num. 8:8), "And a second (sheni) bull, a young of the herd, take for a sin-offering"? If it is to teach that there are two, behold, it already says (Lev. 15:15), "And the Kohen shall make them, one as a sin-offering and one as a burnt-offering." [Rather,] you might have thought that the sin-offering precedes the burnt-offering in every [sacrificial] act, thus the verse says, "And a second bull, a young of the herd, take for a sin-offering." Or, you might have thought that "a second bull, a young of the herd, take as a burnt-offering" comes to teach that the burnt offering precedes the sin offering in every [sacrificial] act. Thus the verse states (Num. 8:12), "and he shall designate one as a sin-offering and one as a burnt offering, to God" (i.e., switching the order). How so? The blood of the sin-offering precedes the blood of the burnt-offering, because it appeases, and the bones of the burnt-offering precede the limbs of the sin-offering, because they are entirely burnt (Zev. 10:2). Rabbi Shimon says, what does it come to teach, "and a second bull, a young of the herd, take as a sin-offering"? You might think that the sin-offering is eaten by the Levites; thus the verse states, "second," as in "second to the burnt-offering." Just as the burnt-offering is not eaten, so too the sin-offering is not eaten. Similarly, said Rabbi Yosei (Ezra 8:35), "The exiles who returned from captivity sacrificed burnt offerings to the God of Israel: twelve bulls for the sake of all Israel, ninety-nine rams^*note that the received text of Ezra 8:35 has "ninety-six rams", seventy-seven lambs, and twelve he-goats as a sin-offering, all these as a burnt-offering to God." Is it possible that the sin-offerings were offered up as burnt-offerings? Rather, just as a sin-offering is not eaten, so too a burnt-offering is not eaten. And so too Rabbi Yehuda would say, is was [as atonement] for idol worship [during the exile] that they brought them.