What is the ruling on one who says: "Allahu akiber" - with a kasra on the baa' - in the opening takbir (takbirat al-ihram)?

The Takbirat al-Ihram (the opening Takbir) is one of the pillars of prayer. Breaking the ba (ب) in it is a linguistic error (lahn) that is not permissible to do intentionally, as it changes the meaning. The prayer becomes invalid by breaking the ba in the Takbirat al-Ihram if the worshipper did it intentionally, or if he was ignorant but able to learn and did not. However, if he did it unintentionally or is unable to learn, his prayer is not invalidated.