Not reciting Al-Fatihah due to Ignorance

Question :

We four persons were in the desert ... and when it came time for the Maghrib prayer, one of us called the Athan and the Iqamah for prayer. During the prayer, in the second Rak'ah, he (the Imam) did not recite Surah Al-Fatihah and one of us drew his attention to this by saying: "Subhan Allah", then the Imam recited Surah Al-Kafirun, but he did not recite Al-Fatihah. At the end of the prayer, he did not perform the prostration of forgetfulness. Then a discussion began regarding this matter. So pelase tell us if it is permissible to make the prostration of forgetfulness or not? And may Allah reward you with every goodness.

Answer:

This prayer which you have mentioned was invalid and must be repeated, due to Surah Al-Fatihah not being recited in the second Rak'ah. They should have reminded him of its recitation by one of them reciting the beginning of Al- Fatihah, then if he had remembered it, he could have recited it while he was still standing and his prayer would have been complete, and even if he had repeated that Rak'ah in which he had not recited Al-Fatihah after the prayer and performed the prostration of forgetfulness at the time of his Taslim, it would have been sufficient for him.

But since he did not recite Surah Al-Fatihah in the second Rak'ah, which is a pillar of the prayer, the Rak'ah was invalid, and since he made the Taslim and finished the prayer and a long time has now passed, the prayer is invalid. As for the prostration of forgetfulness, it does not make up for what was left out, which was Al-Fatihah, since it is one of the pillars of prayer. And Allah is the Granter of success.

Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 180