Rashi on Numbers 8:8/רש"י על במדבר ח׳:ח׳

1ולקחו פר בן בקר AND LET THEM TAKE A YOUNG BULLOCK, — and this was to be a burnt offering, as it is said, (v. 12) “and thou shalt offer the one [as a sin offering] and the other as a burnt offering”, and it (a bullock) was the communal offering prescribed (Numbers 15:24) in case of idol worship committed by the community (here the firstborn are regarded as a community).

2ופר שני AND ANOTHER BULLOCK [THOU SHALT TAKE TO BE A SIN OFFERING] — What is the force of the word שני, the second? (There is no corresponding word אחד at the beginning of the sentence!). But it is used to suggest a comparison between the two sacrifices: How is it in the case of the burnt offering? It is not permitted to be eaten (for it is entirely burnt on the altar)! So, too, is this sin-offering not permitted to be eaten (in spite of being a חטאת חיצונה which as a rule may be eaten; cf. Rashi on Leviticus 9:11). And in this fact (that the חטאת prescribed was not eaten) there is some support in the statement of Torath Cohanim just quoted for his (R. Moses the Preacher’s) view (that the עולה which is paired with it was that enjoined for idolatry committed by the community) (cf. Sifra, Vayikra Dibbura d'Chovah, Chapter 3 4). — I, however, say that it (the fact that this חטאת was not to be eaten) was an occasional (exceptional) decision (הוראת שעה) (contrary to the rule of a חטאת חיצונה, and not because it was intended as a חטאת לע"ז as asserted by R. Moses), for if his view were correct they would have had to offer a goat (as prescribed in Numbers 15:24 not a bullock) as a sin offering to expiate for idolatry together with a bullock for a burnt offering mentioned here.