Midrasch zu Bamidbar 10:4

(Bamidbar 30:2) "And Moses spoke to the heads of the tribes": What is the intent of this? From (Ibid. 10:3) "And when they (the Cohanim) blow (tekiah) with them (the shofaroth) then all the congregation shall gather unto you," and (Ibid. 4) "And if they blow (tekiah) with one, there shall gather unto you the chiefs," we do not know where (they are to gather). It is (therefore) written "tekiah" re the congregation, and "tekiah" re the chiefs. Just as the congregation, (Ibid. 3) "to the door of the tent of meeting," so, the chiefs, at the door of the tent of meeting. — But perhaps all that is first in Scripture (i.e., "congregation") is first in act? It is, therefore, written (Ibid. 30:2) "to the heads of the tribes" (and then to the tribes). Since "spoke" is written in the Torah unqualified (as to whom he is speaking first), and in one instance (i.e., this one) it is specified that the chiefs take precedence to the congregation, it is, likewise, assumed in all such instances that the chiefs take precedence to the congregation. R. Yonathan says: This (derivation) is not necessary. It is already written Shemot 34:31-32) "And Moses called to them, and there returned to him all the chiefs of the congregation, and Moses would speak to them. And after that, all the children of Israel would approach and he would command them, etc." Since "speaking" is written in the Torah unqualified (as to whom he is speaking first), and in one instance it is specified that the chiefs take precedence to the congregation, so, this is assumed in all such instances. If so, why need it be written (here) "to the heads of the tribes"? To indicate that the permitting (i.e., absolution) of vows is through experts alone.