(Bamidbar 31:21) "And Elazar the Cohein said to the men of the host who came to the war: This is the statute of the Torah which the L-rd commanded Moses": It had been forgotten by Moses, our teacher. Because he had succumbed to anger, he succumbed to forgetfulness. R. Elazar says: In three places he succumbed to anger and he succumbed to error: (Vayikra 10:16-17) "and he was wroth with Elazar and Ithamar, the remaining sons of Aaron, saying: Why did you not eat the sin-offering in the holy place?" (Bamidbar 20:10) "And he said to them: Listen, now, you fractious ones!
Shall we bring forth water for you from this rock!" — followed by (11) "And Moses lifted his hand and smote the rock with his staff twice." Here, too, (Ibid. 31:14) "And Moses was wroth with the commanders of the host, the officers of the thousands and the officers of the hundreds, who came from the host of battle" — followed by "And Elazar the Cohein said to the men of the host who came to the war, etc." Moses, our teacher, because he had succumbed to anger, succumbed to forgetfulness.
Others say: Moses authorized Elazar the Cohein to speak, so that when he died they would not say to Elazar: "In your teacher's lifetime you did not speak. Why are you speaking now?" "which the L-rd commanded Moses": He said the thing in the name of its sayer. And thus is it written (Esther 2:22) "And Esther said to the king in the name of Mordecai."