What is the ruling on the Maghrib prayer in which the Imam performed a third prostration in the second rak'ah instead of sitting for the first tashahhud, then sat for the tashahhud without saying "Allahu Akbar" while the congregants were prostrating, and then performed a prostration of forgetfulness (sujood ba'di)?

The Imam's action of returning to the tashahhud (definition) after being prompted by tasbeeh due to adding a third prostration, and then performing the two prostrations of forgetfulness (sujood as-sahw) after the salutation, is correct. An addition of this nature to the prayer necessitates sujood as-sahw. The Imam's return to the tashahhud was without an additional takbeer, as he had already performed it when rising from the second prostration. As for sujood as-sahw, it was performed after the salutation, which is the prescribed method for additions. The congregants who prompted the Imam with tasbeeh acted correctly. As for those among them who prostrated out of forgetfulness or based on an interpretation, their prayer is valid. However, whoever knowingly and intentionally performed the extra prostration, being aware of its prohibition, has invalidated their prayer.