Exodus 4:13

ביד 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)

וַיֹּ֖אמֶר בִּ֣י אֲדֹנָ֑י שְֽׁלַֽח־נָ֖א בְּיַד־תִּשְׁלָֽח׃

And he said, O my Lord, send, I pray thee, by the hand of him whom thou wilt send.