Numbers 1:49

Only thou shall not number the tribe of Levi
That is, along with the other tribes, for it might be numbered by itself, as it afterwards was, ( Numbers 3:43 ) ;

neither take the sum of them among the children of Israel;
which confirms what is before observed: now this being the declared will of God clears Moses from all partiality to his own tribe, he doing nothing but what he had a command of God for it.