Is a caste system an obligated boundary within a nikah?

It is narrated in: In Kitab ul-Athar: >Imam Muhammad (RA) has reported from Umar Radiallahu Anhu saying: “I shall issue an operative order that no girl from some high and recognized family should be given in marriage to another having a lesser status.” What is the authenticity of the following? How should it be understood? Likewise, is such an ruling an obligation? Is it now not permissible for a guardian to accept the marriage of someone with a lower lineage to his daughter?