Rashi on Leviticus 1:7/רש"י על ויקרא א׳:ז׳

1אש … ‎ונתנו AND [THE SONS OF AARON …] SHALL PUT FIRE [ON THE ALTAR] — Although the fire descended from heaven, it was nevertheless a religious duty to bring also some fire of profane origin (Sifra, Vayikra Dibbura d'Nedavah, Chapter 5 10; Yoma 21b; cf. Rashi on Leviticus 10:2 and Note thereon).

2בני אהרן הכהן THE SONS OF AARON THE PRIEST — They shall place the fire on the altar as sons of Aaron the priest: implying that Aaron should always minister in his priestly dignity (i. e. when attired in his priestly garments), consequently if he officiated in the garments of an ordinary priest his service is invalid (Sifra, Vayikra Dibbura d'Nedavah, Chapter 5 9; Zevachim 18a).