Is it obligatory for the congregants to follow the Imam who completed his prayer and said the Taslim, then remembered that he did not recite the Salat al-Ibrahimiyyah and did not invoke blessings upon the Prophet in the last Tashahhud, so he returned to complete his prayer, or is their prayer valid and they are not required to follow him?

Scholars have differed on whether invoking blessings upon the Prophet (peace and blessings be upon him) is a pillar of prayer or merely recommended. The Shafi'is and Hanbalis hold it to be a pillar, while the Hanafis and Malikis consider it recommended.

Accordingly, if the Imam does not consider the Ibrahimiyya prayer (invoking blessings upon Abraham) obligatory, he is not required to repeat the prayer if he omits it, and his prayer is valid. He is not obligated to inform the congregants, nor are the congregants required to follow him if he prostrates for forgetfulness (sujud al-sahw). However, if he considers it obligatory and a pillar, he must return to the prayer to perform it, and the congregants must follow him.