Rashi on Exodus 4:13/רש"י על שמות ד׳:י״ג

1ביד תשלח signifies by the hand of him whom Thou art accustomed to send; viz., Aaron (Exodus Rabbah 3:16). Another explanation is: by the hand of some other person whom Thou wilt be pleased to send — for in the end “I” shall not bring them into the land of Palestine nor shall “I” become their deliverer in the future — and You have many messengers! (Pirkei DeRabbi Eliezer 40)