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’?
Answer:
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