Sforno on Numbers 3:1/ספורנו על במדבר ג׳:א׳

1ואלה תולדות אהרן ומשה ביום דבר ה' אל משה. At the time when G’d set the tribe of Levi apart from the other tribes, some of them having to transport the Tabernacle on its journeys, some to perform service in and around the Tabernacle and to pronounce blessings in G’d’s name. Nadav and Avihu were included in the people described as Levites.