Easier to not have Shia appearing, I would think. However, it would be hard to have Islam never splitting, except if it does not expand as much as it did.
See, maybe more than in Christianism, Islam served as well to exprim political beliefs and different schools, or branchs of Islamic religions were partially due to claims of political factions.
By exemple, Shia Islam is both the expression of first the sucession fight, then the expression of non-Arab Muslims (that were treated by Arab as dhimmis). Shia wasn't originally that much different than Sunna, and it's their lasting division and independent life that managed to make them taking different ways ( abit like Latin and Greek Christianism whom the differences were particularly minor up to XI century, but began to differ more greatly precisly because they were divided).
So, my two cents.
Prevent expand Rashidun Caliphate as much as it did : have them conquer Egypt, Syria, Mesopotamia and Persia (while I would maybe think it would be better to not conquer all of it).
Making that, you managed to have less rebel minorities : granted you have more Christian than anything else, but they won't convert up to some centuries. More interestingly you managed to have no converted minorities (like Berbers that adopted OTL a really "revolutionnary" form of Shia Islam).
With that, you certainly slowed the risk of division.
Eventually, you'll need to have Arab treating non-Arab (if they're Muslims) as superior than dhimmis (I don't ask to have them treated as equals, it would be too hard to reach for the first centuries), maybe a line of energic caliphs whom the shorter than IOTL conquests would give more time to fortify their empire?