Rashi on Numbers 31:21/רש"י על במדבר ל״א:כ״א

1ויאמר 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).

2אשר צוה ה׳ וגו׳ [AND ELEAZAR SAID … THIS IS THE ORDINANCE OF THE LAW] WHICH THE LORD HATH COMMANDED [MOSES] — he associated the decision with his teacher (cf. Sifrei Bamidbar 157:9).