In his book "The Arabs in Antiquity: Their History from the Assyrians to the Umayyads", Professor Jan Retsö writes:
The conflicts were aggravated by the fact that the tribes identified the tribal federation with the Islamic community, so that only members of those tribes belonging to the federation or their clients could be considered real Muslims.
How does Retsö arrive at this conclusion? What historical sources, if any, state that an individual couldn't be considered a real Muslim in the Umayyad Empire before 750 CE if they neither belonged to an Arabian tribe nor were a client of such a tribe?