מדרש על במדבר 31:11

(Numb. 31:6) “Then Moses sent them”: The Holy One, blessed be He, said to Moses (in vs. 2), “’Take vengeance,’ by yourself,” and he sent others? It was simply because he had been raised in Midian, he said, “It is not right for me to oppress those who have done good to me.” The proverb says, “Do not cast a stone into a cistern from which you have drunk water.” But some say that this is not the same Midian in which Moses was raised. As this one is next to Moab and it is [still] desolate until now. And why did He send Phinehas (instead of someone else)? He said, “Whoever has begun a commandment should finish it. [Phinehas] has (according to Numb. 25:11) ‘turned around My wrath’ and smote the Midianite woman. Let him finish the commandment [that he undertook].” (Numb. 31:6, cont.) “And the vessels of the sanctuary (haqodesh)”: This refers to the ark (and its contents), since it is stated (in Numb. 7:9), “because they (i.e., the Children of Kohath) had the service of the holy (haqodesh) objects, which they carried on the shoulder.” R. Johanan said, “These were the priestly garments, in which were the urim and thummim, as stated (in Exod. 29:29), ‘And the sacred (haqodesh) garments of Aaron.’” (Numb. 31:7-8) “And they gather the army against Midian […. …] and they killed [Balaam ben Beor] with the sword”: Now what was he doing there? He had simply gone to receive a reward for the twenty-four thousand from Israel that had fallen through his counsel. And about him is it stated (in Prov. 26:27), “He who digs a pit will fall in it.” (Numb. 31:11-12) “Then they took all the plunder…. And they brought unto Moses and Elazar the Priest”: [This text serves] to show their excellence, for they did not want to take the spoils without permission. Rather they had brought all of it before them; and [only] after that, they took [them with his permission]. (Numb. 31:13) “Then Moses, Elazar the Priest, and the princes of the congregation went out to meet them”: [This text serves] to show Moses' humility, for they all were disciples of his disciples.