Numbers 3:4-9 ESV
But Nadab and Abihu died before the LORD when they offered unauthorized fire before the LORD in the wilderness of Sinai, and they had no children. So Eleazar and Ithamar served as priests in the lifetime of Aaron their father. And the LORD spoke to Moses, saying, “Bring the tribe of Levi near, and set them before Aaron the priest, that they may minister to him. They shall keep guard over him and over the whole congregation before the tent of meeting, as they minister at the tabernacle. They shall guard all the furnishings of the tent of meeting, and keep guard over the people of Israel as they minister at the tabernacle. And you shall give the Levites to Aaron and his sons; they are wholly given to him from among the people of Israel.