Numbers 3:1-4

Muster of the Tribe of Levi. – As Jacob had adopted the two sons of Joseph as his own sons, and thus promoted them to the rank of heads of tribes, the tribe of Levi formed, strictly speaking, the thirteenth tribe of the whole nation, and was excepted from the muster of the twelve tribes who were destined to form the army of Jehovah, because God had chosen it for the service of the sanctuary. Out of this tribe God had not only called Moses to be the deliverer, lawgiver, and leader of His people, but Moses’ brother Aaron, with the sons of the latter, to be the custodians of the sanctuary. And now, lastly, the whole tribe was chosen, in the place of the first-born of all the tribes, to assist the priests in performing the duties of the sanctuary, and was numbered and mustered for this its special calling.
Num_3:1
In order to indicate at the very outset the position which the Levites were to occupy in relation to the priests (viz., Aaron and his descendants), the account of their muster commences not only with the enumeration of the sons of Aaron who were chosen as priests (Num_3:2-4), but with the heading: “These are the generations of Aaron and Moses in the day (i.e., at the time) when Jehovah spake with Moses in Mount Sinai (Num_3:1). The toledoth (see at Gen_2:4) of Moses and Aaron are not only the families which sprang from Aaron and Moses, but the Levitical families generally, which were named after Aaron and Moses, because they were both of them raised into the position of heads or spiritual fathers of the whole tribe, namely, at the time when God spoke to Moses upon Sinai. Understood in this way, the notice as to the time is neither a superfluous repetition, nor introduced with reference to the subsequent numbering of the people in the steppes of Moab (Num_26:57.). Aaron is placed before Moses here (see at Exo_6:26.), not merely as being the elder of the two, but because his sons received the priesthood, whilst the sons of Moses, on the contrary, were classed among the rest of the Levitical families (cf. 1Ch_23:14).
Num_3:2-4
Names of the sons of Aaron, the “anointed priests (see Lev_8:12), whose hand they filled to be priests,” i.e., who were appointed to the priesthood (see at Lev_7:37). On Nadab and Abihu, see Lev_10:1-2. As they had neither of them any children when they were put to death, Eleazar and Ithamar were the only priests “in the sight of Aaron their father,” i.e., during his lifetime. “In the sight of:” as in Gen_11:28.