Mekhilta DeRabbi Yishmael

“And the L–rd spoke to Moses and to Aaron in the land of Egypt, saying”: I might think that both Aaron and Moses were being addressed; it is, therefore, written “And it was on the day that the L–rd spoke to Moses in the land of Egypt”: Only Moses was being addressed and not Aaron. If so, why is it written “to Moses and to Aaron”? We are hereby taught that just as Moses was “included” in the words (i.e., by their being spoken to him), so, Aaron was included in the words (i.e., by his “overhearing” them). And why did He not speak (directly) to him? In deference to Moses. We find Aaron, then, to be excluded from all the (direct) addresses in the Torah except for three (Leviticus 10:8, Numbers 18:1, Numbers 18:8), where it is impossible (to understand otherwise).

Variantly: From “See I have made you an overlord (“elohim”) to Pharaoh,” I would know that only Moses was set up as a judge to Pharaoh. Whence would I derive (the same for) Aaron? (It is, therefore, written (here) “to Moses and to Aaron.” Moses is being likened to Aaron. Just as Moses was a “judge” to Pharaoh, so, Aaron. Just as Moses spoke his words without fear, so, Aaron.