Leviticus 8:5

And Moses said unto the congregation
Having convened them, he opened to them the reason of their being called together, which was not done of himself, but by divine direction: this [is] the thing which the Lord commanded to be done;
namely, what follows, concerning the consecration of Aaron and his sons to be priests, and the investiture of them with that office, attended with various rites and ceremonies to be performed, of which they were to be witnesses.