What is the ruling on saying "bala" (yes, indeed) when the Imam recites "Is not Allah the most just of judges?" (Quran 95:8), and is this statement considered an innovation (bid'ah)?

It is permissible for the congregant to say "balaa" (yes indeed) or "Subhanaka fa balaa" (Glory be to You, yes indeed) when the Imam recites verses such as "Is not He [Allah] able to give life to the dead?" or "Is not Allah the most just of judges?". This is the view of the Maliki and Hanbali schools of thought.

They based their reasoning on a weak hadith (definition) narrated by Musa ibn Abi Aisha, and on the hadith of Hudhayfah concerning the Prophet's prayer at night, where he would glorify Allah, supplicate, and seek refuge when encountering verses of glorification, supplication, and seeking refuge.

Sheikh Ibn 'Uthaymeen holds the view that this is a Sunnah (definition) in supererogatory prayers, especially in the night prayer, but not a Sunnah in obligatory prayers, although it is permissible. This is because it was not narrated that the Prophet (peace be upon him) did so in obligatory prayers. However, his not doing so does not indicate its prohibition, as supplication is not considered human speech that invalidates the prayer.

Ibn 'Uthaymeen affirmed the permissibility for the congregant to say "balaa" when hearing verses that call for it, as long as it does not distract him from listening to the Imam. The hadith mentioned in this regard is weak due to the presence of an unknown narrator.