Numbers 26:57

Ver. 57-62. And these are they that were numbered of the
Levites, after their families
And they were numbered not with the rest of the tribes of Israel, but by themselves, as they were at the first numbering of the tribes; the three principal families of which were, the Gershonite, the Kohathite, and Merarite, so called from the three sons of Levi; but all their sons are not mentioned, of the sons of Gershon only Libni, from whom was the family of the Libnites; not Shimei, because, as Aben Ezra conjectures, either he had no sons, or, if he had, they died without any, and so there was no family from them; and of the sons of Kohath no mention is made of Uzziel, nor of Izhar, but in the Korhites, only of the Hebronite family from Hebron; and of Amram, whose wife Jochebed is spoken of as a daughter of Levi, whom Levi's wife, as Jarchi rightly supplies it, bore to him in Egypt, and which Jochebed was the mother of Aaron, Moses, and Miriam; and it is observed that Aaron had four sons, Nadab, Abihu, Eleazar, and Ithamar, the two first of which died for offering strange fire to the Lord, and the two last were now living: from Merari, another son of Levi, sprang two families, the Mahlite, and the Mushite; and the whole number of the Levites at this time taken was 23,000 males of a month old and upward; so that here was an increase of 1,000 males since the former numbering of them: the reason why they were not numbered with the other tribes was, because they had no part of the land of Israel divided to them, and had no inheritance in it.