As-salamu `alaykum, I attended an iftar in a Ja`fari Shiite association, and then I performed Maghrib and `Isha' Prayers behind a Shiite Imam. Is my Prayer acceptable? They made some differences in Adhan. They also put something (like a stone) in front of them while offering Prayer. Also, in the Prayer itself, they made a few differences.Please let me know whether my Prayer was valid or not. Jazakum Allahu khayran.