What is the ruling on praying behind an Imam who pronounces the letter "ض" (ḍād) sometimes as "ظ" (ẓāʾ) and sometimes as "ض" (ḍād), and what should the worshipper do in a silent prayer where he does not know how the Imam pronounces the "ض" (ḍād)?

Scholars have differed concerning the validity of prayer behind an Imam who pronounces the letter "Ḍād" as "Ẓāʾ" in Al-Fatihah. Some have preferred the view that his imamate is valid. As for silent prayer, the original ruling is that it is valid and is not invalidated except upon verification of a meaning-altering error. If the Imam commits an error that alters the meaning in Al-Fatihah, then prayer behind him is not permissible, whether the prayer is audible or silent.