Answered by Sidi Fadi Qutub Zada
I led the Maghrib prayer as imam. I started to recite Surah Fatihah and the other surah silently; suddenly, I remembered and recited both Surah Fatihah and the other surah loudly, and finished the prayer. I DID NOT do Sajda al-Sahw. Please tell me whether I, and those who followed me in the prayer, should repeat the prayer.
In the name of Allah, Most Compassionate, Most Merciful,
Reciting audibly in the audible prayers (e.g. fajr, maghrib, and isha) is necessary (wajib) upon the imam, and delaying a necessary action in the prayer forgetfully necessitates the forgetfulness prostration (sajda al-sahw).
If the imam forgets to prostrate, then it would be necessary (wajib) upon him and the whole congregation to repeat the prayer. So the imam should notify, to the best of his ability, whoever followed him, that the prayer must be repeated.
And Allah knows best