Numbers 3:4
Now Nadab and Abiu died, without children, when they offered strange fire before the Lord, in the desert of Sinai: and Eleazar and Ithamar performed the priestly office in the presence of Aaron their father.

Numbers 3:6
Bring the tribe of Levi, and make them stand in the sight of Aaron the priest to minister to him, and let them watch,

Numbers 18:2
And take with thee thy brethren also of the tribe of Levi, and the sceptre of thy father, and let them be ready in hand, and minister to thee: but thou and thy sons shall minister in the tabernacle of the testimony.

Jeremiah 33:18
Neither shall there be cut off from the priests and Levites a man before my face to offer holocausts, and to burn sacrifices, and to kill victims continually.

Ezekiel 44:11
They shall be officers in my sanctuary, and doorkeepers of the gates of the house, and ministers to the house: they shall slay the holocausts, and the victims of the people: and they shall stand in their sight, to minister to them.