Does the prayer become void by adding an alif to the Takbirat al-Ihram (Allahu Akbar), and must it be repeated?

An error in the opening takbeer ( Takbeerat al-Ihram ) by adding an alif after the ba (thus saying "Akbar" instead of "Allahu Akbar") invalidates the prayer, because "Akbar" (أكبار) means drums. One must be careful to avoid adding a letter or making a change that alters the meaning. However, one must beware of obsessive doubts (waswasah); the default is the validity of the imams' prayers, and their prayers are not deemed invalid except with certainty. If you are certain of the invalidity of the imam's prayer, then it is not permissible to pray behind him, and you must repeat the prayers you performed behind him. But if it is merely a doubt, do not pay attention to it, as doubt in an act of worship after its completion does not affect its validity.