(Bamidbar 27:18) "And the L-rd said to Moses: Take ("kach) for yourself Joshua the son of Nun": "kach" connotes "seizing," for a friend is acquired only with great difficulty — whence they ruled: One should acquire a friend for himself to learn Scripture with him, to learn Mishnah with him, to eat with him, to drink with him, and to tell his secrets to him. And thus is it written (Koheleth 4:9) "Two are better than one … (12) and the three-fold cord is not soon sundered."
The Holy Spirit responded to Moses: Set up an interpreter for Joshua, and have him ask, expound, and teach rulings in your lifetime, so that Israel not say to him: "In the lifetime of your teacher you did not speak, and now you (suddenly) speak?" Others say: he (Moses) lifted him from the ground and placed him between his knees, and Moses and Israel would lift their heads to hear the voice of Joshua. And this is what it said: "Blessed is the L-rd, who gave Torah to Israel by the hand of Moses our teacher."