Rashi on Numbers 3:6/רש"י על במדבר ג׳:ו׳

1ושרתו אתו [BRING THE TRIBE OF LEVI … BEFORE AARON] THAT THEY MINISTER UNTO HIM — And wherein consists their ministering?

2ושמרו את משמרתו THAT THEY SHALL KEEP HIS CHARGE — Because the care of the Sanctuary, viz., that no stranger should approach it, was committed to his charge, as it is said, (Numbers 18:1) "Thou and thy sons, and thy father’s house with thee, shall bear the iniquity of the sanctuary”, (which Rashi explains to have reference to the iniquity committed by a “stranger” in approaching the holy things), and these Levites aided them — this therefore, was their ministering [alluded to here].