Tea Partiers of the Seventh Century

I've been rereading Eric Schroeder's excellent book Muhammad's People, a chronicle of Islamic history translated from original sources. The following passage describes an incident during the caliphate of Omar ibn al-Khattab (634-644 C.E.):

"Am I a king now? Or a Caliph? Omar once asked Salman the Persian.

If thou tax the land of Believers in money, either little or much, and put the money to any use the Law doth not allow, then thou art a king, and no Caliph of God's Apostle.

By God! said Omar, I know not whether I am a Caliph or a king. And if I am a king, it is a fearful thing."