Comentario sobre Números 31:21

ויאמר אלעזר הכהן וגו׳ AND ELEAZAR THE PRIEST SAID etc. — Because Moses fell into anger (Numbers 31:14) he fell into error: there escaped him the laws concerning the removal of uncleanness absorbed by vessels which have contained the food of heathens. This, too, do you find was the case on the eighth day of the installing of the priests, as it is said, (Leviticus 10:16) “And Moses was angry with Eleazar and Ithamar”, — he fell into anger, he fell into error (cf. 10:16 that he made an error regarding the שעיר החטאת which Aaron refused to eat). Similarly, also, when he said, (Numbers 20:10, Numbers 20:11) “Hear, now, ye rebels”, … “he smote the rock”; it was through his anger that he made the mistake (Sifrei Bamidbar 157:9; cf. Pesachim 66b).

זאת חקת התורה, the instructions you have received from Moses concerning your own purification rites, including being sprinkled with water containing ash from the red heifer, is a decree by the Torah. The institution known as פרה אדומה has as its purpose the purification of people afflicted with ritual impurity due to direct or indirect contact with the dead.

ויאמר אלעזר הכהן אל אנשי הצבא, “Eleazar the High Priest said to the soldiers, etc.“ Moses, personally, conveyed the previous paragraph to the respective commanders concerning the ritual contamination of the people involved. Eleazar addressed the entire people who were all directly involved seeing that all received their respective shares of the loot and had to know how to purify the various vessels or garments. The only loot the commanders of 1000 troops each had taken for themselves were costly garments.