Ramban on Leviticus 8:6/רמב"ן על ויקרא ח׳:ו׳

1AND HE WASHED THEM WITH WATER. It appears likely to me that Scripture tells that he [Moses] washed all of them, but nonetheless it was not done to all of them together. Rather, he washed Aaron and clothed him with his garments and anointed him, and afterwards he washed his sons and anointed them.