What is the ruling on the prayer of the imam and the congregants, and what is incumbent upon them if the imam prostrates three times in one rak'ah and does not perform the prostration of forgetfulness (sujud al-sahw)?

The prayer of the Imam and the congregants is valid, because if the Imam forgets the prostrations of forgetfulness, they are dropped due to a long interval or leaving the mosque. If the Imam deliberately omits the prostrations of forgetfulness, the congregational prayer is not invalidated, because the prostrations of forgetfulness are a Sunnah according to the majority of scholars, and it is permissible to omit them if their place is after the salutation. However, if the Imam deliberately adds a prostration, his prayer is invalidated. As for the congregants who followed him, their prayer is valid if they had a valid interpretation. But if they deliberately added it without a valid interpretation, their prayer is invalidated.