Is my prayer valid if I doubted that I had not recited Al-Fatihah in the Isha congregational prayer with the Imam, so I recited it, then performed the prostration of forgetfulness (sujud al-sahw) after the tasleem?

If the worshipper (ma'mum) doubts his recitation of Surah Al-Fatiha, there are two situations:

1. If the doubt occurs after bowing with the Imam: He must follow the Imam, and when the Imam says the taslim (salaam at the end of prayer), he must perform an additional rak'ah to replace the one in which he doubted. If he does not perform it, his prayer is invalid and he must repeat it. 2. If the doubt occurs before bowing: He must recite Al-Fatiha, and this is the correct view according to the Shafi'i school. He is not required to perform the prostration of forgetfulness (sujud as-sahw) because the Imam bears it on his behalf.

This ruling is based on the fact that Al-Fatiha is a pillar (rukn) for the worshipper (ma'mum), which is the predominant opinion of the Shafi'i school.