Schemuel I 2:27 Midrasch: Shemot Rabbah, Ein Yaakov (Glick Edition), Otzar Midrashim & Bamidbar Rabbah

“He said: Please Lord, send by means of whom You send” (Exodus 4:13).
“He said: Please [bi] Lord [Adonai],³⁴The name of the Lord is spelled alef, dalet, nun, yod. send by means of whom You send.” Rabbi Ḥiyya the Great said: He said before Him: ‘Master of the universe, with me [bi], You seek to deliver the children of Abraham, who established You as the lord [adon] over all Your creations? “Send, please, by means of whom You send.”’³⁵Send whomever You usually send for such tasks. He said before Him: ‘Who is dearer to a person, his nephew or his grandson? You must say, it is his grandson. When you sought to rescue Lot, Abraham’s nephew, You sent angels to rescue him. Abraham’s descendants, who are six hundred thousand, you are sending me to rescue them? Send the angels whom You are accustomed to send.
Another matter: ‘For Hagar the Egyptian, You sent five angels to her; to six hundred thousand descendants of Sarah, it is me You are sending to rescue them?’ The Rabbis say: Did you suppose that Moses was refusing to go? That is not the case. Rather, it was in deference to Aaron, as Moses was saying: ‘Before I arose, Aaron my brother has been prophesying for them in Egypt for eighty years.’ That is what is written: “I made Myself known to them in the land of Egypt” (Ezekiel 20:5). From where [can it be known] that Aaron was prophesying? For it says like this: “A man of God came to Eli, and said to him: So said the Lord: Didn’t I reveal Myself to the house of your father when they were in Egypt, subject to Pharaoh's house? Did I not choose him from all the tribes of Israel to be My priest?”³⁶Implying that Aaron was chosen already in Egypt. (I Samuel 2:27–28). Moses said: ‘Will I now encroach upon my brother and upset him?’ For that reason he did not wish to go. Immediately, “the wrath of the Lord was enflamed against Moses” (Exodus 4:14).

We are taught that R. Simeon b. Jachai said: "Come and see how the Israelites are beloved by the Holy One, praised be He! Wherever they were exiled the Shechina accompanied them. When they were exiled in Egypt, the Shechina was with them, as is said (I Sam. 2, 27) Did I rveal Myself unto the house of thy father, when they were in Egypt? When they were exiled into Babylon the Shechina was with them, as is said (Is. 43, 14) For your sake I was sent to Babylon. And in the future, when they will be redeemed, the Shechina will also return with them, as is said (Deut. 30, 3) The Lord thy God will return thy captivity, and have mercy upon thee, V'heshib (He win bring back you) is not said, but V'Shab (He will return). We infer from this that the Holy One, praised be He! will return with the Israelites from the exile."

This entire passage, Aaron prophesied to them in Egypt, and from where do we know that Aaron prophesied to them in Egypt? As it says "And a man of God came to Eli etc." (I Samuel 2:27) and it is written "And I chose him etc." (Psalms 105:26). Therefore Moses said: Aaron my brother was prophesying to them all these years, now I will go to him. The Holy One, blessed be He, said to Moses: Moses! He rejoices in your greatness. When? When he was anointing Aaron with the anointing oil, the oil bubbled and flowed down Aaron’s beard. Moses thought it was on his own beard, as it says "Like the goodly oil upon the head etc." (Psalms 133:2). What is "upon the beard"? One beard! Did Aaron have two beards? Rather, it flowed down on Aaron's beard as if it was on Moses' beard (see Vayikra Rabbah 3). The Holy One, blessed be He, said to Moses: By your life, any greatness that I bestow upon Aaron is through you that I bestow upon him, as it says "And you shall bring near to yourself Aaron your brother, and this is the thing that you shall do to them" (Exodus 28:1, 29:1). Therefore it says "And these are the generations of Aaron and Moses."