Kommentar zu Bamidbar 3:1

ואלה תולדת אהרן ומשה AND THESE ARE THE OFFSPRING OF AARON AND MOSES — But it mentions only the sons of Aaron! But they also are called the sons of Moses because he taught them the Torah. This tells us that whoever teaches the Torah to the son of his fellow man Scripture regards it to him as though he had begotten him (Sanhedrin 19b).

NOW THESE ARE THE GENERATIONS OF AARON AND MOSES IN THE DAY THAT THE ETERNAL SPOKE WITH MOSES IN MOUNT SINAI. “And yet [in the following verse] Scripture only enumerates the sons of Aaron, [as it is said, And these are the names of the sons of Aaron! If so, why does it say here and Moses?] They [were indeed the sons of Aaron, but] they are called the generations of Moses because he taught them Torah. This establishes the principle that if one teaches the Torah to one’s fellow man,¹⁴³In our Rashi: “the son of his fellow man.” Scripture accounts it to him as though he had begotten him. In the day that the Eternal spoke to Moses did these children [of Aaron] become his [Moses’], because he taught them then what he learned from the mouth of the Almighty.” This is Rashi’s language.
However, the expression in the day that the Eternal spoke to Moses is intended [not to convey the idea mentioned by Rashi, but] only to say that these were [Aaron’s four] sons [namely, Nadab and Abihu, Eleazar and Ithamar, as mentioned in Verse 2] in the day that the Eternal spoke with Moses in Mount Sinai, and then Scripture states [in the following verses] that Nadab and Abihu died, and that therefore they¹⁴⁴I.e., Aaron and Moses. — Ramban is following the verse which speaks of the children as the generations of Aaron and Moses, for the reason that Rashi explained, but he is of course referring to Aaron’s sons. have no children now except Eleazar and Ithamar. Now Scripture mentions this in order to state that in the day that the Eternal spoke with Moses in Mount Sinai these [sons of Aaron] were chosen to be anointed, and their anointing shall be to them for an everlasting priesthood,¹⁴⁵Exodus 40:15. but as for the rest of the tribe [of Levi], Moses was not commanded that they be chosen [for service in the Tabernacle] until now. And the reason why He mentioned the generations of Aaron and Moses at this place is that since He had completed the census of all Israel by their generations and by their fathers’ houses,⁵⁸Verses 20, 22, etc. and He now wanted to mention the generations of the tribe of Levi, He began with the leaders of that tribe.
According to the plain meaning of Scripture, the sense of [the expression] the generations of Aaron and Moses is that the sons of Aaron were anointed priests separated from [the rest of] the tribe [of Levi] so that they be most holy,¹⁴⁶I Chronicles 23:13. and the generations of Moses were the family of the Amramites which He mentions further on, since of the Amramites there were only the children of Moses,¹⁴⁷Further, Verse 27. Of Amram’s two sons, the sons of the first, Aaron, are already mentioned above; thus the verse must be speaking of Moses’ children only. and they are counted among the Levites. This [verse] is thus similar to that which it is written, The sons of Amram: Aaron and Moses; and Aaron was separated, that he should be sanctified as most holy.¹⁴⁶I Chronicles 23:13. But as for Moses the man of G-d, his sons are named among the tribe of Levi.¹⁴⁸I Chronicles 23:14. And the homiletic interpretation [which Rashi mentioned, that the generations of Aaron are also attributed to Moses because he taught them Torah], the Rabbis based upon [the fact that] Scripture does not state “and these are the names of the sons of Moses,” as it does with the sons of Aaron [stating in Verse 3: These are the names of the sons of Aaron — therefore the Rabbis explained that the verse says: Now these are the generations of Aaron and Moses] to allude to [the principle that] the sons of Aaron are also the generations of Moses because he taught them Torah, for it is the way of the Torah to explain and to allude [to spiritual truths].

ואלה תולדות אהרן ומשה ביום דבר ה' אל משה. At the time when G’d set the tribe of Levi apart from the other tribes, some of them having to transport the Tabernacle on its journeys, some to perform service in and around the Tabernacle and to pronounce blessings in G’d’s name. Nadav and Avihu were included in the people described as Levites.