Answered by Sidi Fadi Qutub Zada
If an imam forgetfully recites the Fatihah and a surah quietly in one rak’a within a ”loud” prayer, and then performs the sajdah sahw at the end, having realized that he made a mistake, is his prayer valid? Is it a wajib that such a person has missed? What about the followers? Is there any way in which they can prompt the imam, seeing that he continues his quiet recitation until going into ruku’?
In the name of Allah, Most Compassionate, Most Merciful,
Wa alaykum assalam,
Reading audibly in a loud prayer is a necessary (wajib) action in the prayer; leaving this action forgetfully necessitates a forgetfulness prostration (sujud sahw) [Nur al-Idah]. Therefore, the imam’s prayer is valid and correct without dislike (karâha). The followers’ prayer is also valid.
The followers can prompt the imam by tasbeeh (saying: subhan Allah), by coughing, or by clearing their throats.
And Allah knows best
Fadi Qutub