Numbers 1:47

45So all those who were counted of the children of Israel by their fathers’ houses, from twenty years old and upward, all who were able to go out to war in Israel—

46all those who were counted were six hundred three thousand five hundred fifty.

47But the Levites after the tribe of their fathers were not counted among them.

48For the LORD spoke to Moses, saying,

49“Only the tribe of Levi you shall not count, neither shall you take a census of them among the children of Israel;