With no PoDs prior to 841 CE, how much can the further growth of Islam, as a world religion, be prevented (in terms of population of adherents)? If we can agree on changes that would do this, what would be the larger affects of said changes?
To get us started, the latter 9th century was a hard time for the Abbassid Caliphate as is OTL, so it should give is stuff to work with. Alternatively, we might just focus on keeping future large centers of population (India, Indonesia, etc) from seeing Islamic growth in future centuries.