Arabia would be the main Islamic area, with small communities in the Middle East and Persia, around the Red Sea, the Horn of Africa, and if ITTL's Muslims do really well distant areas like India or Indonesia.
However ITTL's Islam would be far different than the Islam of IOTL. For example the Quran was compiled into its final form in part because of the need to have a standardized Arabic text for the diverse people of the Caliphate. Without that the Quran would be compiled in a different way, possibly leading to suras that were removed IOTL staying in or suras that exist IOTL being lost. In addition the Quran would be in a much different script, since Arabic society wouldn't have evolved into one where writing was very important without the conquests. This would allow for variations in how the Quran was read to exist (with different people pronouncing different words differently for instance).
Another major change would be that Islam would be far more diverse. After Muhammad's death several tribes revolted, attempting to go back to their previous religions or with tribal leaders declaring themselves prophets. Such a trend would no doubt reemerge if the early Muslims were unable to expand. As a result Islam might be used to refer to one specific set of beliefs on the Arabian Peninsula, or perhaps it would be used as an umbrella term for a variety of similar sects that emerged in that area in the 7th and 8th centuries.