Dearborn. 40,000 muslims in a city around 100,000 population.
I should clarify that. Wikipedia shows the population of "Arab Americans" as 40,000 in Dearborn. The article is clear in stating that "most" of the Arabs in the area are actually Lebanese Christians who immigrated early in the 20th century to work the automotive industry. However, a better understanding is that there is a significant muslim population there.
Some estimates are that the muslims make up 33,000 of the 40,000 identified as "Arab American".