What is the ruling on the prayer of the Imam and the congregants if the Imam forgot to say the takbir for the second prostration in the first rak'ah, and the congregants glorified God to alert him, but he thought it was an addition, then completed his prayer and performed the prostration of forgetfulness, then realized the omission and added an extra rak'ah? And must the Imam repeat the prayer? And what is the ruling on the prayer of the congregant who stood up to alert the Imam then returned and completed his prayer with the congregation?

The 'takbirat al-intiqal' (transitioning takbirs) are considered sunnah (definition) by the majority of scholars and obligatory by the Hanbalis. The Imam's prayer is valid. The first rak'ah is invalidated for the congregants who did not perform the second prostration, and they must perform an additional rak'ah in its place. The prayer of both the Imam and the congregants is valid if they stood for an extra rak'ah out of ignorance of the legal ruling, based on the hadith (definition) where the Prophet (peace be upon him) stood up for a fifth rak'ah out of forgetfulness. The speech of a congregant for the benefit of the prayer does not invalidate it according to many scholars, and their prayer is valid if they were ignorant or had an interpretation. However, the more cautious approach is for all congregants to repeat the prayer.