What is the ruling on praying behind an Imam who does not recite the Quran proficiently, and who is among the people of innovation, such as performing congregational Istighfar aloud immediately after the prayer, knowing that he inherited the Imamate and did not pray in the mosque before his father's illness?

Voicing supplication for forgiveness (Istighfar) and remembrance (Dhikr) aloud after prayer is not an innovation (bid‘ah), as it is supported by the Sunnah and was reported by Ibn Abbas to have existed during the time of the Prophet, peace and blessings be upon him. For this reason, many jurists (fuqaha) have affirmed it. However, performing it in a collective, unified voice is an innovation, as it was not reported from the Prophet, peace and blessings be upon him, or his companions.