Assalamualaikum ustaz. If an imam forgets to recite al-Fatihah in the second rakaat but just recite the surah. Although the makmum has signalled the mistake of the imam, the imam just continues his surah recitation and then bowed (rukuk). What is the status of the prayer?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
Prayer has its own rukun (requisites). It is divided into the fi’li requisites which are related to actions and qauli requisites which is related to recitation and qalbi requisites and maknawi requisites. All of these requisites are obligatory to be performed in order for the prayer to be valid.
Reciting al-Fatihah is one of the qauli requisites. The prayer is invalid if one didn’t recite al-Fatihah. This is in accordance with the statement of the Prophet PBUH:
لاَ صَلاَةَ لِمَنْ لَمْ يَقْرَأْ فِيهَا بِفَاتِحَةِ الْكِتَابِ
“He who does not recite Fatihat al-Kitab (Surah al-Fatihah) is not credited with having observed prayer.” 
In explaining this hadith, Imam al-Nawawi said: “Reciting al-Fatihah for those who are capable to do so is an obligation and requisite where it cannot be translated into another language other than Arabic and cannot be replaced with any other recitation from the Quran. It is also obligatory to be recited in all obligatory and sunnah prayers.” 
This means that prayer is invalid if al-Fatihah is not recited when one is praying. If a person forgets to recite al-Fatihah, the obligation is not abrogated for him. This opinion is agreed by the jumhur of Syafie scholars. This is also stated by Imam al-Nawawi. 
In this issue, there are three situations for a person who forgets to recite al-Fatihah:
- If he forgets to recite al-Fatihah and remembered when he was bowing (rukuk), then he should stand up and recite al-Fatihah.
- If he remembered in the next rakaat, then the Ftaihah recitation is a replacement of the forgotten al-Fatihah. Before salam, he must stand and add another rakaat as well as perform the sahwi prostration.
- If he remembered after giving salam and not a lot of time has passed, then he must stand and add another rakaat replacing the one he had forgotten, followed by sahwi prostration. However, if a long time has passed and then he remembered that he forgot to recite al-Fatihah in one of his rakaat, then he has to perform the prayer again. 
For an imam who forgets to recite al-Fatihah, his prayer is not invalid but he has to add another rakaat before salam. The reason is the rakaat which he has forgotten to recite Fatihah is not considered. Thus, before salam, he must stand and add another rakaat for the rakaat that he forgot to recite al-Fatihah. However, if he doesn’t stand and add another rakaat after salam, then his prayer is invalid and he has to pray again.
For the makmum, they have two choices if the imam forgot to recite al-Fatihah:
- Continue to follow the imam with the hope and assumption that the imam will add another rakaat for the one he forgot to recite al-Fatihah. If the imam didn’t add the rakaat at the end of the prayer, then makmum can set the intention of mufaraqah and replace the rakaat himself.
- The makmum can set the intention to mufaraqah and continue the prayer alone.
It is best to follow the imam. This is so that it will not cause confusion and havoc. If the imam didn’t replace the rakaat before salam, it is permissible for the makmum to set the intention to mufaraqah.
Let us all learn and study our religion especially in matters of the requisites of Islam so that we can practise our religion in line with the shariah.
 Narrated by Ibnu Majah (828)
 See al-Majmu’ Syarh al-Muhazzab, 3/283