Is the prayer of one who prayed 'Isha behind an Imam valid, where the Imam performed a long Qunut in the second rak'ah, then said the taslim, thinking it was the Fajr prayer, and then the worshipper stood up and completed two more rak'ahs and performed the prostration of forgetfulness (sujud al-sahw) after being alerted by the congregants?

If the Imam intended to pray Fajr and did not intend to pray Isha, he must repeat the Isha prayer. However, if he intended to pray Isha, then forgot and performed the Qunut, thinking he was praying Fajr, his prayer is valid, and he must perform the prostration of forgetfulness (sujud al-sahw). The prayers of the congregants are valid in both cases, and they are not required to repeat them, because the Imam's intention is hidden from them, just like his ritual impurity. Jurists, such as the Shafi'is, have explicitly stated that repetition is not required in such cases. Their following the Imam in Qunut does not harm the prayer because prolonging the standing after bowing (i'tidal) does not invalidate the prayer, as stated in Sahih Al-Bukhari regarding the equality of the prayer's pillars, which indicates that the i'tidal is a lengthy pillar.