מילון על בבא בתרא 240:2

ראשי המטות [AND MOSES SPAKE TO] THE HEADS OF THE TRIBES OF THE CHILDREN OF ISRAEL] — This does not mean that he spoke only to the princes of the children of Israel and not to the people also, but that he showed respect to the princes by teaching them first and that afterwards he taught the children of Israel. This explanation seems to assume that this was the general method of instruction, but from what Biblical verse may we infer that this was so in the case of all addresses, and that this was no exceptional case? Because it says, (Exodus 34:31—32) “[And Moses called unto them], and Aaron and all the princes of the congregation returned unto him and Moses spake unto them; and afterwards all the children of Israel came nigh, [and he gave them all the commandments which the Lord had spoken to him in Mount Sinai]”. But if this be no exceptional case, what reason is there for Scripture going out of its way to state it here? Because by specially stating that the laws about vows were taught in the first instance to the princes and afterwards to the Israelites, it intends to teach that the annulling of vows is really to be effected by one person who is expert in the Law, and that only if there be no such expert, may they be annulled by three ordinary persons. But you may perhaps say that the statement does mean that Moses spoke this section only to the princes of the children of Israel, and that Moses did not speak to them also, and that consequently nothing about the expert and laymen can be inferred! But it is stated here, “This is the word”, and it is stated in the passage dealing with “animals slaughtered outside the sanctuary” (Leviticus 17:2) “This is the word”. What is the case there? It was addressed to Aaron and to his sons and to all the children of Israel, as it is said there, “Speak unto Aaron [and unto his sons and unto all the children of Israel]”! So, too, this was addressed to all of them (and the reason why it is stated here is to tell us that the annulling of vows etc., as above) (cf. Sifrei Bamidbar 153:1; Nedarim 78a).