Deuteronomy 18:6-8

(6-8) And if a Levite come. — The Levites with the priests were to receive forty-eight cities in Israel, with the suburbs (Numbers 35:7). There was as yet no provision made by which all could serve in turn at the tabernacle. When David divided them all into courses, priests, Levites, singers (and porters?) alike, there was no longer any need for this provision. The institutions of David prove its antiquity. The only case in history that illustrates it is that of the child Samuel. His father, Elkanah, was a descendant of Korah. He dwelt in Mount Ephraim, and came up to Shiloh year by year. But Samuel was dedicated by his mother to perpetual service there, and as long as the tabernacle continued in Shiloh, the child Samuel “ministered to the Lord before Eli the priest” — not as a priest, but as a Levite in attendance upon the priests.