Midrash su Numeri 18:8

“And Hashem said to Moses and to Aaron in the land of Egypt, saying, ‘I hear that the the word was to Aaron and to Moses. When it says, ‘It happens on the day Hashem spoke to Moses (in the land of Egypt)’ (Exodus 6) - it was to Moses and not to Aaron. If so why does it teach, ’to Moses and to Aaron'? Except it teaches that just as that Moses was crowned for leadership, thus also Aaron was crowned for leadership. For what reason did he not speak with him? Because of his respect for Moses. Aaron is found excluded from all of the leadership that is in the Torah except from three places, from which it is possible.

(Bamidbar 18:8) "And the L-rd spoke to Aaron": I would think (the intent is) that the speaking was to Aaron; it is, therefore, written (Ibid. 17:5) "A sign for the children of Israel … as the L-rd spoke to Moses about him" (Aaron, viz., that only he and his sons are to be Cohanim), whereby we are apprised that the speaking was to Moses, to tell to Aaron. (Bamidbar, Ibid.) "And I, behold, (I have given to you") with joy, (the twenty-four priestly gifts.) These are the words of R. Yishmael — whereupon his disciples said to him: But master, it is written (Bereshit 6:17) "And I, behold, shall bring a flood of water, etc." Are we, then to assume that this was a joy to Him? He answered: When His angerers go lost from the world, it is a joy to Him. And thus is it written (Proverbs 11:10) "When the righteous prosper, the city exults, and when the wicked perish there is rejoicing." And (Psalms 3:9-10) "You have broken the teeth of the wicked. Deliverance is the L-rd's. Upon Your people are Your blessings, selah." And (Ibid. 10:16) "The L-rd is King for ever and ever. Nations have gone lost from His land." And (Ibid. 104:35) "Sinners will end from the earth, and the wicked will be no more. Bless the L-rd, O my soul, Hallelukah!" R. Nathan said to him: I will add to your words: "And I" — willingly; "behold" — with joy. And thus is it written (Shemot 4:14) "Behold, he (Aaron) is going out to meet you (Moses; and when he sees you, he will rejoice in his heart."