Bava Batra 120 - Not to Intermarry between Tribes
Upon their entry into the Land of Israel, the Jews were commanded to marry only within tribes: "... Any daughter who inherit a legacy shall become the wife of someone from a family of her father's tribe ..."
This was true, however, only for the first generation after entry, because it is introduced with ".. this is the thing that God has commanded ...", which teaches that this "thing" applies only in this generation. The reason was to encourage the Jews who entered the Land to conquer all of it from the Canaanite inhabitants - and not get it through intermarriage.
The daughters of Tzelophchad were given the same commandment, but for them it was just good advice - because they were wise , expounders of the Torah , and righteous . Nevertheless, they followed it . Moreover, a miracle happened to them: they were rejuvenated and married righteous, fitting husbands.
Art: Gerrit - Wedding Dance in a Tavern