Midrash su Deuteronomio 31:7

It is written (Deut. 31, 7) Thou must go with this people, etc., and it is written (Ib. 23) Thou shalt bring. R. Jochanan said: Moses said to Joshua: "You and the elders shall rule over them;" but the Holy One, praised be He! said: "Thou (alone) shalt rule over them, because there must be one ruler to a generation, and not two rulers."

(Ibid. 28) "And charge (tzav) Joshua": "charging" (tzivui) is prompting to zeal, as it is written (Ibid. 31:7) "And Moses called to Joshua and he said to him before the eyes of all of Israel: 'Be strong and firm'": "Be strong" in Torah, "and be firm" in good deeds.

R. Nathan says: Moses was saddened that one of his sons did not stand (in his place) — whereupon the L-rd said to him: Are the sons of your brother Aaron not like your sons? He, too, that I appoint over Israel will go and stand at Elazar's door (to learn from him). To what may this be compared? A king had a son fit for royalty and he took the (governance of the) kingdom from him and gave it to his lover, telling him: Though I gave you greatness, go and stand at my son's door. Likewise, the Holy One Blessed be He said to Joshua: Go and stand at the door of the sons of Elazar, viz. (Bamidbar 27:21) "And before Elazar the Cohein shall he (Joshua) stand." At that time the strength of Moses increased and he strengthened Joshua before the eyes of all of Israel, as it is written (Devarim 31:7) "And Moses called to Joshua and he said to him before the eyes of all of Israel: 'Be strong and firm!'" Behold, the people that I am giving you are still kids, still infants. Do not be censorious of what they do, for neither was their Master, viz. (Hoshea 11:1) "For Israel was a youth and I loved him," and (Song of Songs 1:8) "And graze your kids by the dwellings of the shepherds" — Take them into the shepherds' tents. Variantly: (Ibid.) "beikvei hatzon" — They were trodden down by the heel (ekev). "And graze your kids".