Sifrei Bamidbar 140

(Bamidbar 27:18) "And the L-rd said to Moses: Take for yourself Joshua the son of Nun": ("for yourself") what is in your heart. Take for yourself him (Joshua), who you know to be worthy (of this high station), as per (Mishlei 27:18) "the watcher of the fig-tree will eat its fruit, and the keeper (Joshua) of his master (Moses) will be honored." (Bamidbar, Ibid.) "a man who has spirit in him": one who can accommodate himself to the spirit of each individual. "and place your hand upon him": He said to him: Give Joshua an interpreter to ask and to expound and to issue rulings in your lifetime, so that when you die Israel not say: In his master's lifetime he did not rule, and now he rules! — whereupon he raised him from the ground (the place of disciples) and sat him next to him on the (judge's) bench. R. Nathan says: When Joshua entered, he (Moses) would silence the interpreter until he had come in and sat in his place.

(Bamidbar 27:20) "And you shall place (some) of your glory (i.e., shining of countenance) upon him": and not all of your glory — whence we derive "The face of Moses was like the face of the sun; the face of Joshua was like the face of the moon."