Deuteronomy 10:8

10:8 At that time the Lord set apart the tribe of Levi to carry the ark of the Lord’s covenant, to stand before the Lord to serve him, and to formulate blessings in his name, as they do to this very day.

Deuteronomy 18:5

18:5 For the Lord your God has chosen them and their sons from all your tribes to stand and serve in his name permanently.