Many Rishonim are of the opinion that the Yishmieilim are not classified as idol worshippers since they believe in Hashem and pray to one G-d. Therefore the Rambam writes that their wine is not considered yayin nesech and although it is prohibited to drink, it is mutar b’hanaah.
Even though it is forbidden to enter the houses of worship of the nations but the Yishmieilim’s place of worship (i.e., a mosque) is different. Many Achronim write that it is permitted to make a shul in a mosque like R' Yitzchak Elchonon Spector writes, among other Achronim.
Based on their words, today we daven at the Me’aras Hamachpeila, Kever Shmuel Hanavi and other places where the Yishmieilim have established a mosque. However, the Klausenberger Rebbe as well as the Tzitz Eliezer oppose davening in a mosque.
