You're right, this one's pretty easy; just make sure that the Muslims never conquer Egypt, and that the Umayyads (or an analogous ATL equivalent of them) remain in charge and continue their policy of keeping Islam the religion of the Arab ruling elite, with only a few non-Arabs being allowed to convert.
Then make sure that the Muslims lose their conquests in Persia (again, this shouldn't be too hard), and Islam will mainly be restricted to the Arabian Peninsula, and, depending on how much of these territories the Muslims managed to retain, Syria, Mesopotamia, and Palestine.