What is the ruling on the prayer of an Imam who doubted the number of rak'ahs (three or four) in the fourth rak'ah, then stood up for a fifth, whereupon the congregants glorified Allah (said "Subhan Allah") to him, so he immediately sat down without performing the prostration of forgetfulness (sujood as-sahw), believing that he was not required to perform sujood as-sahw and that the congregants had not followed him in standing up for the fifth (rak'ah)?

What you did by standing up when you doubted whether it was the third or fourth [rak'ah] is correct, because the Prophet, peace and blessings be upon him, commanded anyone who doubts in their prayer to build upon the lesser number. And what the congregants did by saying "Subhan Allah" to you, as well as what you did by returning [to sitting] when they said "Subhan Allah" to you, all of that is correct and in accordance with the Sunnah. The error was in your omission of the prostration of forgetfulness (sujūd as-sahw), because it was legislated to mend an excess that would invalidate the prayer if done intentionally. Scholars have differed on whether prostrating for something whose intentional commission would invalidate the prayer is obligatory or a Sunnah. If we say it is obligatory, then whoever omitted it out of forgetfulness or ignorance until a long time passed, there is nothing upon them. And the congregants' following the Imam in the prostration of forgetfulness is obligatory, even if they did not forget.