Is the congregational prayer valid if the imam sat for the tashahhud in the third rak'ah and then prayed only two rak'ahs by mistake, and he did not perform the prostration of forgetfulness (sujood al-sahw), and the congregants remembered this more than five minutes after the prayer, while the imam denied it?

Congregational prayer and the Imam's prayer are valid because the addition was due to forgetfulness (sahw) and was a superfluous act (laghw). They were required to perform the prostration of forgetfulness (sujood al-sahw), which is waived for them according to some scholars since they remembered after leaving the mosque. However, the Malikis hold that the prostration should be performed whenever it is remembered, even if it is after a year.